Dart: local-variable constructor-call typing not seeded (svc.method() unresolved)

[carlos-alm]

Context

Follow-up to #2319, discovered while verifying that fix's resolution-benchmark improvement.

#2319 added typeMap population for Dart's explicitly-typed field declarations and this.field constructor-shorthand params, moving the resolution benchmark's receiver-typed category from 0/8 to 4/8 (50% recall; overall dart recall 61.9% → 81.0%). The remaining 4/8 false negatives are all local-variable method calls in tests/benchmarks/resolution/fixtures/dart/main.dart:

var repo = UserRepository();
var svc = UserService(repo);   // <-- svc's type is never seeded

svc.createUser('1', 'Alice', 'alice@example.com');  // unresolved
svc.getUser('1');                                    // unresolved
svc.removeUser('2');                                 // unresolved
svc.summary();                                        // unresolved

svc's type (UserService) is knowable with full certainty from its constructor-call initializer (UserService(repo)) — the same "assign a constructor call to a local variable" pattern every other language extractor in this codebase already treats as a distinct, high-confidence (1.0), always-implemented category (e.g. JS/TS's handleVarDeclaratorTypeMap for const p = new Ctor(), and its Rust mirror handle_var_declarator_type_map). Dart's src/extractors/dart.ts and crates/codegraph-core/src/extractors/dart.rs have no equivalent for local_variable_declaration (WASM grammar) / initialized_variable_definition (native grammar) nodes.

This was deliberately left out of #2319's scope — that issue's own scope explicitly excluded "type inference from initializers without an explicit type annotation (var x = Foo();)" as a blanket exclusion (not limited to fields), to keep the change narrowly scoped to fields/constructor-shorthand params.

Suggested fix direction

Add a Dart-specific local_variable_declaration / initialized_variable_definition handler (mirroring JS's handleVarDeclaratorTypeMap's constructor-call branch, confidence 1.0) in both src/extractors/dart.ts and crates/codegraph-core/src/extractors/dart.rs, seeding typeMap[varName] = ctorType when the initializer is a bare/keyword-less constructor call (var svc = UserService(repo);). Verify the exact grammar shapes for both engines before implementing — confirmed via tree-sitter-dart parsing during #2319's investigation:

• WASM (npm tree-sitter-dart 1.x): local_variable_declaration -> initialized_variable_definition -> inferred_type(var) identifier("svc") = identifier("UserService") selector("(repo)").
• Native (crates.io tree-sitter-dart 0.2): local_variable_declaration (initialized_variable_definition name: (identifier) value: (call_expression function: (identifier) arguments: (...))).

After this lands, Dart's resolution-benchmark receiver-typed category should move from 4/8 to 8/8 (100% recall), and overall Dart recall from 81.0% to 100%.