chore: migrate AST chunking tests to vitest

[jamesrisberg]

Summary

Builds on #449

Replaces the standalone test-ast-chunking.mjs script (823 lines, custom check() harness) with a proper vitest test file. The script ran outside the test pipeline and was invisible to CI.

Changes

• Deleted test-ast-chunking.mjs
• Added test/ast-chunking.test.ts — 12 integration tests covering areas not already tested by test/ast.test.ts:
  • mergeBreakPoints score precedence and position sorting
  • AST vs regex split comparison on a 30-function file
  • Markdown regression (identical chunks in auto vs regex mode)
  • "regex" strategy bypass, no-filepath fallback, small file → single chunk
  • chunkDocumentWithBreakPoints equivalence with chunkDocument
  • Break point score assertions: TS export=90, Python class=100, Go type=80, Rust enum=80

What was intentionally dropped

• Performance benchmarks — timing loops that printed to stdout, not assertions
• Real collection scanner — CLI dev tool for manual profiling against real codebases
• Duplicate tests — language detection, per-language breakpoints, and error handling already covered by test/ast.test.ts

Test plan

• 38/38 tests pass (26 existing + 12 new)
• No source code changes — test-only PR
• Net −651 lines