test(parsers): pin loader structure with golden-file tests

[mrsibe]

What does this PR do?

Adds test/parsers.test.ts, a golden-file suite that parses the committed fixtures in test/fixtures/ and asserts the structure each loader emits — page counts, page boundaries, heading outlines, and offset arithmetic — rather than the extracted prose.

Why?

test/fixtures/ already shipped sample.pdf, sample.docx and sample.html, but no test under test/ exercised a loader at all. The provenance work in #66/#67 is about to start trusting DocumentLoadResult.structure, and there was no regression net if a loader silently stopped emitting it.

Related issue

Fixes #64

What changed?

• test/parsers.test.ts — 8 tests covering PDF pages, DOCX sections, Markdown sections, HTML structure, preserveStructure: false, and FileParserService dispatch. Assertions are structural (counts, level outlines, offset monotonicity/containment/slice-consistency), not full-text equality.
• Fixtures — reused sample.pdf/sample.docx/sample.html; added three small ones where the existing set could not exercise a required case:

  Fixture	Loader it exercises	Why
  sample.pdf	PdfLoader	existing single-page smoke fixture; text + page box
  multipage.pdf	PdfLoader	3 pages, so page boundaries/offsets are tested across pages
  sample.docx	DocxLoader	existing heading-less fixture; structure must still be present
  headings.docx	DocxLoader	minimal OOXML with Heading1/Heading2 styles
  sample.md	MarkdownLoader	one h1 + two h2
  sample.html	WebLoader	existing Readability smoke fixture

• package.json — npm test now passes --experimental-transform-types. Node's strip-only type support rejects the enum in src/shared/utils/logger.ts, which is on every loader's import path; transforming lets the tests import the real loaders instead of a test-only copy. The rationale is recorded under the //test key.

How was this tested?

• npm test — 92 tests pass (84 existing + 8 new); no network required.
• npm run typecheck — node/web/test projects all pass.
• npx eslint test/parsers.test.ts — clean.
• npx prettier --check on the touched files — clean.
• Negative check: temporarily forced PdfLoader to return structure: undefined; 3 parser tests failed as intended, then reverted. This verifies the "a loader that stops populating structure fails" acceptance criterion.

Screenshots / recordings

Not applicable (test-only change).

Checklist

• I have reviewed my own changes.
• npm run typecheck passes.
• npm run build passes. (not run — no production code changed; CI will run it)
• I have tested the affected user workflow. (test suite)
• I have not included unrelated changes.
• I have updated documentation when necessary.

Desktop / build changes

• Not applicable