feat(pipeline): add IDML/PDF ingest stage to the build pipeline

[PAMulligan]

Summary

Adds a new ingest ETL stage to the InDesign pipeline that extracts a source document (.idml/.pdf) into a generator-ready bundle on disk:

• ir.json — the intermediate IR the theme generator consumes
• assets/ — image bytes pulled from the source, named to match the generator's planAssets() output
• assets.manifest.json — per-frame provenance + unresolved-asset warnings

Why

Embedded IDML image bytes previously had no automatic path into a generated theme — generate-theme could only stage images from a loose --asset-dir, and nothing extracted the bytes the IDML zip carries. This stage resolves them once and stages them under the exact filenames generateTheme() references, so the bundle feeds straight into generation:

flavian pipeline ingest brochure.idml --out-dir build/brochure
flavian pipeline indesign build/brochure/ir.json --asset-dir build/brochure/assets

What's in the stage

• Extract — parse to IR; pull image bytes (IDML zip via extractIdmlAssets, PDF via the parse-pdf image cache).
• Transform — resolveAssets / buildIngestBundle join IR image frames to an href → bytes resolver (pure, source-agnostic), computing staged paths via the generator's own planAssets so they line up byte-for-byte downstream.
• Load — writeIngestBundle writes ir.json + assets/ + assets.manifest.json.

Runnable via pnpm pipeline:ingest, flavian pipeline ingest, and the flavian-ingest bin. Adopts the in-progress idml-assets.js helper (and its unit test).

Test plan

• New ingest.test.mjs — transform, manifest shape, and an end-to-end IDML run whose staged paths line up with generateTheme() (3/3)
• assets-idml.test.mjs — embedded-asset extraction (2/2)
• InDesign smoke test scripts/indesign-fse/smoke-test.mjs ✓
• InDesign fixtures test tests/fixtures/indesign/fixtures.test.mjs (3/3)
• End-to-end CLI: embedded IDML image → ingest → generate → image bytes land in theme/assets/ automatically
• commitlint passes on the commit

The full @flavian/pipeline suite is green except two pre-existing snapshot tests that only fail on a Windows (CRLF) checkout — .gitattributes stores them as LF (git ls-files --eol → i/lf), so they pass on the Ubuntu CI runner.

Closes #117

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