[repo-assist] perf: reuse singleton JavaScriptMetricsAnalyzer instance in analyzeFile()

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Summary

Both sibling analyzers, typescriptAnalyzer.ts and tsxAnalyzer.ts, already reuse a module-level singleton instance in their analyzeFile() static factory to avoid allocating a new analyzer object (and its underlying Tree-sitter parser wiring) on every call — this was called out explicitly in code comments as a deliberate optimization. javascriptAnalyzer.ts was missed when that pattern was introduced and still did return new JavaScriptMetricsAnalyzer().analyzeFunctions(sourceText); on every invocation.

This PR applies the identical, already-proven singleton pattern to javascriptAnalyzer.ts for consistency, eliminating an unnecessary object allocation on every provideCodeLenses call for JavaScript files.

Why it is safe

JsLikeMetricsAnalyzer (the shared base class) resets its mutable per-analysis state at the start of each top-level function analysis using a save/restore pattern, so reusing a singleton instance across calls is safe — exactly the same reasoning already documented and relied upon by typescriptAnalyzer.ts/tsxAnalyzer.ts.

Trade-offs

None identified. This is a pure internal refactor with no behavioral or API changes.

Test Status

• npm run compile: passes
• npm run lint: passes
• npm run test:unit: 236 tests passing, coverage 98.71/94.99/99.1/98.71 (thresholds 95/88/97/95)
• npm test (vscode-test): not run in this sandbox (no network access to download the VS Code test binary) — expected/known limitation; CI has network access and will verify.

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