Add generated mnemonic_builders.odin (inst_<mnem>/emit_<mnem> typed overload sets) for arm32, arm64, mips, riscv, ppc, ppc_vle, rsp, mos6502 and mos65816, matching the existing x86 builders. Each is produced by a per-arch tools/gen_mnemonic_builders.odin that walks ENCODE_FORMS and maps operand types to typed params + op_* constructors.
Anchor every generator's output via #directory so regeneration is CWD-independent; previously the bare "mnemonic_builders.odin" path wrote to the current directory and misfired when run from the repo root.
Wire a --builders task into build.lua (folded into 'all', covered by --idempotent, enforced by the structural invariants) and document it in the README.
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