feat(extract): tree-sitter-based Dart extraction with source_location line anchors

[cornwe19]

What

Dart is currently the one mainstream language whose extractor emits source_location: None on every node and edge — extract_dart is a ~520-line regex extractor predating the generic tree-sitter framework, so Dart symbols can't be jumped to or read by line range downstream, unlike Java/Kotlin/Swift/etc.

This PR makes tree-sitter the primary Dart path, using the community tree-sitter-dart grammar as a new optional [dart] extra (prebuilt wheels for win/macOS/Linux, x86+arm):

• AST-driven with line anchors: declarations (classes, mixins, enums, extension types, extensions, typedefs), imports/exports, part of redirection, annotations, and functions/factory constructors all come from the AST with L{n} anchors on nodes and edges.
• Proven heuristics retained: the Flutter-framework sweeps (Bloc events/emissions, Riverpod providers, navigation routes) and Dart 3 pattern-destructured variables — which the 0.1.x grammar still parses with ERROR nodes — stay on the existing regexes, now line-anchored via match offsets and scoped by the AST.
• Regex fallback preserved verbatim: without the extra installed, extraction falls back to the previous implementation unchanged (_extract_dart_regex) — the same optional-grammar pattern as extract_pascal (feat(extract): add Pascal/Delphi and Lazarus IDE support #781). No new required dependency.

Fixes along the way

The AST also closes two regex false-positive classes:

• Declarations inside string literals (e.g. lint-test fixtures with code in '''...''') no longer produce phantom class/function nodes.
• Multi-line named/optional parameter lists (required String id, sits at the 0–2-space indent the variable regex scans) no longer leak parameters as variable nodes, including a case that produced labels containing newlines.

Validation

• All existing tests/test_dart.py expectations pass on both paths (grammar installed and absent); new tests cover anchor stamping, part of redirection with anchors, the extension family (named/anonymous/generic), named/optional parameters, factory vs named constructors, and generators.
• Ran against a large production Flutter codebase (2,780 .dart files): 100% of locally-defined nodes and all edges anchored, 100% anchor spot-check (2,547 sampled symbols found on their anchored line), node counts within 0.5% of the regex baseline (the delta is the string-literal phantoms the AST correctly drops), and the tree-sitter path ran slightly faster than the regex path (~5s vs ~5s for the whole repo, same order).
• Error recovery verified: files the grammar can't fully parse still extract (localized ERROR nodes), and any walker exception falls back to the regex extractor per file, so a grammar gap can never cost a file entirely.

uv.lock regenerated with uv lock (adds tree-sitter-dart to the all/dart extras; the surrounding marker churn is current uv re-normalizing environment markers).

🤖 Generated with Claude Code