feat(elixir): add tree-sitter language support

[dclausen]

Summary

• Add Elixir language detection, tree-sitter parser loading, queries, and extractor/provider wiring.
• Add provider-owned container hooks so Elixir defmodule ownership is visible without broadening shared AST container rules.
• Register Elixir in the worker parser path and invalidate stale parse-worker cache output.

Validation

• npm run build
• npm test -- worker-pool.test.ts tree-sitter-languages.test.ts incremental-parse-cache.test.ts
• npm test -- tree-sitter-languages.test.ts language-skip.test.ts framework-detection.test.ts call-extraction.test.ts
• npx tsc --noEmit
• git diff --check
• gitnexus detect-changes --scope all --repo GitNexus

Risk / rollback

• Elixir parser support is additive and marked experimental.
• Parse-cache schema bump forces old worker output to be rebuilt after install.

[vercel]

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