fix(treesitter): recognize JavaScript arrow function definitions

[odvcencio]

Summary

• supplement the resolved JavaScript tags query with arrow functions bound by variable declarators
• preserve existing inferred JavaScript definitions and calls
• cover the exported arrow definition, ordinary function definition, and both call references

Evidence

• exact baseline: 1b47566
• baseline focused test fails because fetchUser is absent while normalize remains indexed
• 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
• implementation and assertions come from the preserved corrected response; integration changed only hunk placement and final-newline mechanics

Validation

• go test ./pkg/lang/treesitter -run ^TestJavaScriptArrowFunctionDefinition$ -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
• commit-range gitleaks scan: no findings