fix(treesitter): index Rust type aliases

[odvcencio]

Summary

• supplement the inferred Rust tags query with type_item alias definitions
• preserve all existing inferred Rust definitions and references
• cover the alias, ordinary function definition, and call reference

Evidence

• exact baseline: 3e2cdb5
• bounded matrix probe 1, Python async function and calls, passed
• probe 2 stopped the matrix: Rust UserId alias was absent while normalize and main definitions and the normalize call remained
• the Rust AST query for type_item and type_identifier matches UserId directly
• live indexing changes the sample from 2 to 3 symbols while preserving the main -> normalize call edge
• production and regression logic come from preserved response 1; integration changed only malformed hunk and new-file mechanics

Validation

• go test ./pkg/lang/treesitter -run ^TestRustTypeAliasDefinition$ -count=20
• focused Ruby, JavaScript, TypeScript, and Rust tag tests
• 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