feat(extraction): add GDScript language support

[Plumvery]

Summary

Adds GDScript (.gd, Godot Engine) as a supported language on the wasm extraction path, following the shape of previous language additions (Nix #1190, ArkTS #648).

• Extractor (src/extraction/languages/gdscript.ts, ~100 lines): functions with typed signatures, _init constructors, inner classes with methods, the full var/const/@export/@onready variable family, signals, enums with members, static func detection, and call edges through call / attribute_call / base_call.
• Grammar: PrestonKnopp/tree-sitter-gdscript v6.1.0 (MIT), vendored as an ABI-15 wasm rebuilt from upstream source (tree-sitter-cli 0.25.10 generate + build --wasm, emscripten/emsdk:4.0.4), external scanner (indentation) included — the same approach as the Nix wasm.

Grammar shapes that needed care

• The var/const family names its target via a name-typed child, not identifier, so the core's generic variable fallback can't read them. A visitNode hook creates variable/constant nodes itself, then walks initializers so calls inside them (preload(...), Foo.new()) are captured.
• func _init(...) (constructor_definition) has no name field — resolveName supplies the conventional _init. enumerator names its identifier via left.
• static on functions is a named static_keyword child with no field (the var statements carry it via a field) — a child scan covers both.
• signal foo(a, b) extracts as a property carrying the parameter list, so connect()-heavy scripts expose their signal surface in the graph.
• obj.method(args) parses as attribute(identifier, attribute_call(...)) — attribute_call joins callTypes and the core's namedChild(0) callee fallback yields the bare method name (name-match resolution, same as self/this receivers elsewhere).

Validation

scripts/add-lang/check-grammar.mjs: ABI 15, 20/20 clean parses, heap-safe under multi-grammar reuse (the Lua ABI-13 corruption scenario this script exists for does not occur).

scripts/add-lang/verify-extraction.mjs criteria on two corpora (all critical and soft checks pass):

Corpus	.gd files	nodes	edges	index time
godotengine/godot-demo-projects (public)	461	5,216	6,783	3.7s
Private Godot 4 game	138	2,996	11,484	1.8s

Kind spread on the public corpus: 2,027 functions / 184 methods / 45 classes / 141 signals→properties / 39 enums + 130 members / 1,570 variables / 422 constants — symbol density well above the 1-per-file soft bar.

Tests: 4 new extraction describe-blocks + detection/support assertions in __tests__/extraction.test.ts; full vitest suite passes locally (all pre-existing tests unaffected).

Not included (deliberately small)

• No kernel routing (stays on the wasm path, like the other recent language additions).
• No preload/load import-edge resolution to res:// paths (calls are captured; resolving them to file nodes would follow the Luau instance-path require pattern and can come as a follow-up).
• No README language-grid SVG icon (asset pipeline is maintainer-side).

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