fix(treesitter): capture TypeScript arrow function definitions

[odvcencio]

Summary

• supplement the resolved TypeScript tags query with arrow functions bound by variable declarators
• preserve all existing inferred and curated captures
• cover the exported arrow definition, ordinary function definition, and both call references

Evidence

• exact baseline: b13ba83
• baseline focused test fails because fetchUser is absent from symbols
• feature focused test passes for 20 consecutive runs
• live indexing changes the sample from 1 to 2 symbols and resolves the fetchUser -> normalize call edge
• production logic is copied verbatim from the preserved reviewed response; only the test plumbing was adapted to existing Canopy APIs

Validation

• go test ./pkg/lang/treesitter -run ^TestTypeScriptArrowFunctionDefinition$ -count=20
• go test ./pkg/lang/treesitter -count=1
• go test -race ./pkg/lang/treesitter -count=1
• go test ./pkg/index ./pkg/xref ./cmd/canopy -count=1
• go test ./... -count=1
• go test -race ./... -count=1
• git diff --check
• gitleaks on both changed files: no findings