rexcode: re-house ISA packages under core:rexcode/isa/<arch>

Move all ten ISA packages (x86, arm32, arm64, mips, riscv, ppc, ppc_vle,
rsp, mos6502, mos65816) from core/rexcode/<arch> to core/rexcode/isa/<arch>,
so the import pattern is now `import "core:rexcode/isa/x86"`. The shared
core stays at core:rexcode/isa.

Mechanical: relative `import "../isa"` / "../../isa" -> absolute
"core:rexcode/isa" (the only path that survives the move; the "../" and
"../.." self/generated imports move with their packages). build.lua now
builds paths as <root>/isa/<name>; stale `cd <arch>` hints in the verify
tools and the doc.odin paths updated.

WASM stays at core/rexcode/wasm for now -- it is an IR, not an ISA, and
will move under the forthcoming core:rexcode/ir once that layer lands.

All 10 arches gen/builders/check/test green; import core:rexcode/isa/x86
verified working; wasm still compiles.

core/rexcode/build.lua

@@ -202,7 +202,8 @@ end
 -- ----------------------------------------------------------------------------
 local ODIN, ROOT, OUT  -- set in main
 
-local function pkg(isa, sub) return ROOT .. "/" .. isa.name .. (sub and ("/"..sub) or "") end
+-- ISA packages live under <root>/isa/<name> (the shared isa package is <root>/isa).
+local function pkg(isa, sub) return ROOT .. "/isa/" .. isa.name .. (sub and ("/"..sub) or "") end
 
 local function odin_run(target)  return q(ODIN).." run "..q(target).." -out:"..q(OUT) end
 local function odin_check(target)return q(ODIN).." check "..q(target).." -no-entry-point" end

core/rexcode/doc.odin

@@ -48,15 +48,15 @@ emits committed binary blobs that the library `#load`s into `@(rodata)` at
 compile time — no table is built during a normal library build:
 
 ```sh
-odin run <arch>/tablegen            # ENCODING_TABLE -> generated Odin + <arch>/tables.odin
-odin run <arch>/tablegen/generated  # -> <arch>/tables/<arch>.*.bin
+odin run isa/<arch>/tablegen            # ENCODING_TABLE -> generated Odin + tables.odin
+odin run isa/<arch>/tablegen/generated  # -> isa/<arch>/tables/<arch>.*.bin
 ```
 Regenerate after editing `ENCODING_TABLE`. See `docs/table_migration.md`.
 
 ## Usage
 
 ```odin
-import "x86"
+import x86 "core:rexcode/isa/x86"
 
 instructions := []x86.Instruction{
 	x86.inst_r_r(.MOV, x86.RAX, x86.RDI),
@@ -179,21 +179,21 @@ Tasks: `--gen` (table metaprograms), `--builders` (regenerate each ISA's
 Each package has its own test suite:
 
 ```sh
-odin run x86/tests
-odin run arm32/tests
-odin run arm64/tests
-odin run mips/tests
-odin run mos6502/tests
-odin run mos65816/tests
-odin run ppc/tests
-odin run ppc_vle/tests
-odin run riscv/tests
-odin run rsp/tests
+odin run isa/x86/tests
+odin run isa/arm32/tests
+odin run isa/arm64/tests
+odin run isa/mips/tests
+odin run isa/mos6502/tests
+odin run isa/mos65816/tests
+odin run isa/ppc/tests
+odin run isa/ppc_vle/tests
+odin run isa/riscv/tests
+odin run isa/rsp/tests
 ```
 
 ## Verification harnesses
 
-Each arch has a verification harness under `<arch>/tools/`:
+Each arch has a verification harness under `isa/<arch>/tools/`:
 - `dump_verify_input.odin` — emits the per-entry hex/asm manifest.
 - `verify_against_<tool>.*` — runs the canonical external assembler/
   disassembler and compares. LLVM-mc for the seven modern archs, plus
@@ -203,24 +203,27 @@ Each arch has a verification harness under `<arch>/tools/`:
 
 ```
 rexcode/
-	isa/                # shared core: labels, status, print framework, label-inference
+	isa/                # shared ISA core: labels, status, print framework, label-inference
+		x86/            # x86-64 / i386
+		arm32/          # AArch32
+		arm64/          # AArch64
+		mips/           # MIPS (R1..R6 + ASEs + coprocessors)
+		mos6502/        # NMOS 6502 family
+		mos65816/       # W65C816S
+		ppc/            # PowerPC (Power ISA 3.1)
+		ppc_vle/        # Freescale VLE (sibling of ppc)
+		riscv/          # RISC-V
+		rsp/            # N64 RSP
+	wasm/               # WebAssembly (an IR; destined for ir/wasm once the IR layer settles)
 	docs/               # cross-arch design + per-arch design docs
-	x86/                # x86-64 / i386
-	arm32/              # AArch32
-	arm64/              # AArch64
-	mips/               # MIPS (R1..R6 + ASEs + coprocessors)
-	mos6502/            # NMOS 6502 family
-	mos65816/           # W65C816S
-	ppc/                # PowerPC (Power ISA 3.1)
-	ppc_vle/            # Freescale VLE (sibling of ppc)
-	riscv/              # RISC-V
-	rsp/                # N64 RSP
 ```
 
-Per-package layout (canonical, enforced by the cross-arch contract):
+Each ISA is imported as `core:rexcode/isa/<arch>` (e.g. `core:rexcode/isa/x86`); the
+shared core is `core:rexcode/isa`. Per-package layout (canonical, enforced by the
+cross-arch contract):
 
 ```
-<arch>/
+isa/<arch>/
 	encoder.odin         # encode() — two-pass, label/reloc-aware
 	decoder.odin         # decode()
 	printer.odin         # sb/sbln/print/println/aprint/aprintln/tprint/tprintln/bprint/bprintln/fprint/fprintln/wprint/wprintln

core/rexcode/isa/arm32/decoder.odin

@@ -2,7 +2,7 @@
 
 package rexcode_arm32
 
-import "../isa"
+import "core:rexcode/isa"
 
 // =============================================================================
 // AArch32 DECODER

core/rexcode/isa/arm32/encoding_types.odin

@@ -2,7 +2,7 @@
 
 package rexcode_arm32
 
-import "../isa"
+import "core:rexcode/isa"
 
 // =============================================================================
 // AArch32 ENCODING FUNDAMENTALS

core/rexcode/isa/arm32/printer.odin

@@ -7,7 +7,7 @@ import "core:fmt"
 import "core:io"
 import "core:os"
 import "core:reflect"
-import "../isa"
+import "core:rexcode/isa"
 
 // =============================================================================
 // AArch32 PRINTER

core/rexcode/isa/arm32/tools/dump_verify_input.odin

@@ -17,7 +17,7 @@ package main
 //   /tmp/rexcode_arm32_t32w.hex / _meta.txt   -- T32 32-bit (high half first)
 //   /tmp/rexcode_arm32_t16.hex  / _meta.txt   -- T16 16-bit (2-byte halfword)
 //
-// Run:  cd arm32 && odin run tools/dump_verify_input.odin -file
+// Run:  cd isa/arm32 && odin run tools/dump_verify_input.odin -file
 
 import "core:fmt"
 import "core:os"
@@ -87,7 +87,7 @@ main :: proc() {
 	fmt.println("  llvm-mc --disassemble -triple=arm-none-eabi   -mattr=+armv8,+neon,+vfp4,+crc,+crypto < /tmp/rexcode_arm32_a32.hex   > /tmp/rexcode_arm32_a32_llvm.txt   2>&1")
 	fmt.println("  llvm-mc --disassemble -triple=thumbv8-none-eabi -mattr=+v8.1m.main,+mve.fp,+fp.dp,+vfp4 < /tmp/rexcode_arm32_t32w.hex > /tmp/rexcode_arm32_t32w_llvm.txt 2>&1")
 	fmt.println("  llvm-mc --disassemble -triple=thumbv8-none-eabi -mattr=+v8.1m.main,+mve.fp,+fp.dp,+vfp4 < /tmp/rexcode_arm32_t16.hex  > /tmp/rexcode_arm32_t16_llvm.txt  2>&1")
-	fmt.println("Then:  cd arm32 && odin run tools/verify_against_llvm.odin -file")
+	fmt.println("Then:  cd isa/arm32 && odin run tools/verify_against_llvm.odin -file")
 }
 
 // Fill in safe non-zero values for operand fields so LLVM can decode without

core/rexcode/isa/arm64/decoder.odin

@@ -2,7 +2,7 @@
 
 package rexcode_arm64
 
-import "../isa"
+import "core:rexcode/isa"
 
 // =============================================================================
 // AArch64 DECODER

core/rexcode/isa/arm64/encoding_types.odin

@@ -2,7 +2,7 @@
 
 package rexcode_arm64
 
-import "../isa"
+import "core:rexcode/isa"
 
 // =============================================================================
 // AArch64 ENCODING FUNDAMENTALS

core/rexcode/isa/arm64/printer.odin

@@ -6,7 +6,7 @@ import "core:strings"
 import "core:reflect"
 import "core:os"
 import "core:io"
-import "../isa"
+import "core:rexcode/isa"
 
 // =============================================================================
 // AArch64 PRINTER

core/rexcode/isa/arm64/tests/pipeline_smoke.odin

@@ -11,7 +11,7 @@ package rexcode_arm64_tests
 import "core:fmt"
 import "core:os"
 import a "../"
-import "../../isa"
+import "core:rexcode/isa"
 
 @(private="file") rpasses   := 0
 @(private="file") rfailures := 0

core/rexcode/isa/arm64/tools/dump_verify_input.odin

@@ -15,7 +15,7 @@ package main
 //
 // Then verify_against_llvm.odin reads meta + llvm output and reports mismatches.
 //
-// Run:  cd arm64 && odin run tools/dump_verify_input.odin -file
+// Run:  cd isa/arm64 && odin run tools/dump_verify_input.odin -file
 
 import "core:fmt"
 import "core:os"
@@ -59,5 +59,5 @@ main :: proc() {
 	fmt.println("      < /tmp/rexcode_aarch64_input.hex \\")
 	fmt.println("      > /tmp/rexcode_aarch64_llvm.txt 2>&1")
 	fmt.println("Then:")
-	fmt.println("  cd arm64 && odin run tools/verify_against_llvm.odin -file")
+	fmt.println("  cd isa/arm64 && odin run tools/verify_against_llvm.odin -file")
 }

core/rexcode/isa/arm64/tools/verify_against_llvm.odin

@@ -19,7 +19,7 @@ package main
 //   /tmp/rexcode_aarch64_verify_report.txt   -- full report
 //   /tmp/rexcode_aarch64_verify_mismatches.txt -- just the mismatches
 //
-// Run:  cd arm64 && odin run tools/verify_against_llvm.odin -file
+// Run:  cd isa/arm64 && odin run tools/verify_against_llvm.odin -file
 
 import "core:fmt"
 import "core:os"

core/rexcode/isa/mips/decoder.odin

@@ -2,7 +2,7 @@
 
 package rexcode_mips
 
-import "../isa"
+import "core:rexcode/isa"
 
 // =============================================================================
 // MIPS DECODER

core/rexcode/isa/mips/encoding_types.odin

@@ -2,7 +2,7 @@
 
 package rexcode_mips
 
-import "../isa"
+import "core:rexcode/isa"
 
 // =============================================================================
 // MIPS ENCODING FUNDAMENTALS

core/rexcode/isa/mips/printer.odin

@@ -6,7 +6,7 @@ import "core:strings"
 import "core:reflect"
 import "core:os"
 import "core:io"
-import "../isa"
+import "core:rexcode/isa"
 
 // =============================================================================
 // MIPS PRINTER

core/rexcode/isa/mips/tools/dump_verify_input.odin

@@ -13,7 +13,7 @@ package main
 // MIPS u32 instruction words go on the wire big-endian; llvm-mc's
 // `-triple=mips` consumes hex bytes in that order, so we emit byte 3,2,1,0.
 //
-// Run:  cd mips && odin run tools/dump_verify_input.odin -file
+// Run:  cd isa/mips && odin run tools/dump_verify_input.odin -file
 
 import "core:fmt"
 import "core:os"

core/rexcode/isa/mos6502/decoder.odin

@@ -2,7 +2,7 @@
 
 package rexcode_mos6502
 
-import "../isa"
+import "core:rexcode/isa"
 
 // =============================================================================
 // MOS 6502 DECODER