[tannevaled]

Marshal MRI-4.0.6 conformance: Data + Regexp encoding

Advances core/marshal/dump_spec.rb and core/marshal/load_spec.rb against MRI 4.0.6. Two self-contained clusters, entirely within internal/vm/marshal_codec.go plus a new test file.

Data (Data.define, Ruby 3.2+ immutable value objects)

• Dump via the S struct container (member name/value pairs), the same encoding MRI uses, using the real class name (never a #name override).
• Load via memberDefNames, reconstructing a frozen instance (Data is immutable regardless of the freeze: kwarg).
• Fixes all Data examples: dump (a Data, extended, overridden name, object links) and load (a Data, extended, frozen).

Regexp encoding + option byte

• Record the compiled Regexp's encoding the way MRI does: bare (no encoding ivar) for a binary source, :E => true/false for UTF-8/US-ASCII, a named :encoding ivar otherwise.
• Emit the option byte's FIXEDENCODING (0x10) and NOENCODING (0x20) bits; treat an ASCII-incompatible source (UTF-16/UTF-32) as fixed even when its bytes are ASCII-only.
• Restore both on load, so a Regexp round-trips byte-for-byte, while keeping a loaded /café/ equal to the literal.
• Fixes the four encoding examples in dump_spec (binary, ascii-compatible, UTF-8, another-encoding).

Conformance deltas

• dump_spec: 162 → 170 pass (+8)
• load_spec: 210 → 213 pass (+3)
• Total: +11 examples, no regressions.

Out of scope (object-model / lexer / non-marshal, per task boundaries)

• String/Array/Hash/Struct instance variables and extend/subclass identity — core value types don't carry ivars or a class override.
• Regexp subclass identity and ivars — UserRegexp.new yields a plain Regexp.
• Symbol per-symbol encoding (Symbol#encoding).
• Time zone/offset fidelity (in time.go), Exception message re-encoding (in the constructor).
• Array#== uses identity/eql? for custom-== elements (core comparison, not marshal) — this alone accounts for the three "…and with proc" / "loads a user-marshaled object" load failures, whose loaded objects are structurally correct.
• Wrapped-C-pointer (_dump_data/_load_data) and Random — need builtins.

Verification

• go test ./internal/vm/ and go test ./... green.
• wasm build (GOOS=js GOARCH=wasm) OK.
• gofmt -l empty on changed files.
• MERGED coverage 100.0% (-coverpkg over ./internal/...).

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