feat(scanner): add Dart and Flutter dependency analysis

[jasontitus]

What does this PR do?

Adds first-class Dart and Flutter dependency analysis across the scanner pipeline:

• recognizes .dart files in the canonical language registry
• extracts Dart imports, exports, conditional imports, part directives, top-level functions, and methods through embedded ast-grep rules
• discovers dependencies and dev dependencies from pubspec.yaml
• resolves relative Dart URIs and local package: URIs across multi-package workspaces
• fails closed when package names are ambiguous
• ignores generated .dart_tool content
• renders Dart dependencies and documents the new language support

The pinned ast-grep 0.42.1 release already includes its Dart parser, so this does not require a separate grammar download or release-workflow change.

Type of change

• Bug fix
• New feature
• New language support
• Documentation
• Other (describe below)

Checklist

• I've tested this locally with go build && ./codemap .
• I've read CONTRIBUTING.md (for new language support)
• I've updated documentation if needed

Verification

• go test ./...
• go test -race ./scanner ./render
• go vet ./...
• staticcheck ./...
• git diff --check

Tested against a real multi-package Flutter repository: 166 Dart files, 1,178 functions, 366 resolved dependency edges. This covered same-package imports, cross-package workspace imports, generated localization sources, tests, and vendored Flutter packages.

Additional notes

The existing language-contribution instructions reference the retired tree-sitter query layout. This implementation follows the current embedded ast-grep rule architecture used by scanner/astgrep.go and scanner/sg-rules.