Add binary serialization of the parsed AST (JRuby support, step 2)

[soutaro]

Stacked on #2998 (step 1). Review/merge that first; this PR's diff is against it.

Why

For JRuby, the parser runs inside WebAssembly and the AST has to come back to Ruby without the Ruby C API. This PR adds that bridge: the parser serializes the AST to a compact binary buffer, and pure-Ruby code rebuilds the same RBS::AST objects on the other side.

Both ends are generated from config.yml, right alongside the existing C→Ruby translation (ast_translation.c), so they can't drift apart.

What's here

• src/serialize.c (rbs_serialize_node) — generated C encoder that walks the C AST into the binary format. Self-contained C, so it compiles into both the extension and the .wasm.
• lib/rbs/wasm/serialization_schema.rb — generated table describing every node.
• lib/rbs/wasm/deserializer.rb — pure-Ruby decoder, the counterpart of ast_translation.c. Locations are rebuilt through the public RBS::Location API, so the same decoder works whether Location is C-backed (CRuby) or pure Ruby (JRuby, step 3).
• docs/wasm_serialization.md — the wire format.
• The template generator now also emits .rb files (with a Ruby-style header).

How it's validated (on CRuby, no WASM needed)

The extension exposes _parse_signature_to_bytes / _parse_type_to_bytes / _parse_method_type_to_bytes. test/rbs/wasm/serialization_test.rb parses each input twice — once through the normal C→Ruby translation, once through serialize→deserialize — and asserts the two trees are deeply identical, down to locations and string encodings (stricter than RBS ==, which ignores location/comment).

Coverage: the entire bundled RBS corpus (core + stdlib + sig, ~340 files) plus type and method-type batteries. All green locally (4 tests, 389 assertions).

This de-risks the hardest part of the JRuby work entirely on CRuby, before any WASM/JVM is involved.

Notes

• Found a latent bug while doing this: rbs_hash_t never updates its length field (unlike rbs_node_list_t, which does in rbs_node_list_append). Nothing currently reads it, so it's harmless today, but the serializer can't trust it — it counts hash entries by walking the list. Worth a separate fix to rbs_hash_set if you'd like.
• Heads up, unrelated to this PR: a local steep check currently dies with RBS::DuplicatedMethodDefinitionError: Module#ruby2_keywords — it's defined in both core/module.rbs:1701 and steep/patch.rbs:4. That blocks the type-checker from even building its environment here.

Next step (#step 3): load rbs_parser.wasm with Chicory on JRuby, feed its output to this deserializer, and branch lib/rbs.rb on RUBY_ENGINE.

https://claude.ai/code/session_01LTveMt3NLbYHEboXuzAKpA

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