terminal/snapshot: kaitai verification

Describe the complete version 1 snapshot format with a Kaitai schema and make every golden fixture self-describing for automatic discovery. Add a verifier that compiles the schema, parses all fixtures, and checks record checksums, checkpoint digests, and cross-record invariants.

Preserve Kaitai metadata when generating fixture candidates and provide the compiler and Python runtime dependencies through the development shell. Keep the mode registry portable to Kaitai JavaScript targets so the complete fixture also works in the web IDE.

nix/devShell.nix

@@ -59,6 +59,7 @@
   zlib,
   alejandra,
   jq,
+  kaitai-struct-compiler,
   minisign,
   pandoc,
   pinact,
@@ -85,6 +86,11 @@
   gi_typelib_path = import ./build-support/gi-typelib-path.nix {
     inherit pkgs lib stdenv;
   };
+  python = python3.withPackages (python-pkgs: [
+    python-pkgs.blake3
+    python-pkgs.kaitaistruct
+    python-pkgs.ucs-detect
+  ]);
 in
   mkShell {
     name = "ghostty";
@@ -116,9 +122,10 @@ in
 
         # Testing
         parallel
-        python3
+        python
         vttest
         hyperfine
+        kaitai-struct-compiler
 
         # wasm
         wabt
@@ -138,11 +145,6 @@ in
         blueprint-compiler
         libadwaita
         gtk4
-
-        # Python packages
-        (python3.withPackages (python-pkgs: [
-          python-pkgs.ucs-detect
-        ]))
       ]
       ++ lib.optionals stdenv.hostPlatform.isLinux [
         # My nix shell environment installs the non-interactive version
@@ -242,6 +244,6 @@ in
         # We need to remove "xcrun" from the PATH. It is injected by
         # some dependency but we need to rely on system Xcode tools
         export PATH=$(echo "$PATH" | awk -v RS=: -v ORS=: '$0 !~ /xcrun/ || $0 == "/usr/bin" {print}' | sed 's/:$//')
-        export PATH="/opt/homebrew/opt/llvm/bin:/opt/homebrew/bin:/usr/local/opt/llvm/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:$PATH"
+        export PATH="${python}/bin:/opt/homebrew/opt/llvm/bin:/opt/homebrew/bin:/usr/local/opt/llvm/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:$PATH"
       '');
   }