feat(pipeline): add asset-staging stage and repair #152 merge fallout

[PAMulligan]

Summary

Lands a working stage-assets ETL stage and repairs the breakage that PR #152's conflict-resolution merge left on main.

The stage (Extract → Transform → Load)

stage-assets resolves an InDesign source's image bytes and stages them into a generator-ready bundle — the one thing neither the parser, the ingest stage, nor the generator did before:

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

Because the staged filenames come from the generator's own planAssets(), the bundle feeds straight into generation:

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

Pure, source-agnostic transform (resolveAssets / buildAssetBundle) + runAssetStage orchestration. Runnable via pnpm pipeline:stage-assets and the flavian-stage-assets bin. This also makes the previously-orphaned idml-assets.js (merged by #152) functional.

Repairs from the #152 merge (tracked in #154)

PR #152 merged after a conflict resolution that left main degraded — CI stayed green because nothing exercised the broken paths:

• Dangling barrel export — src/indesign/index.js re-exported resolveAssets, buildIngestBundle, writeIngestBundle, runIngest from ./ingest/index.js, which exports none of them, so importing the package's ./indesign barrel threw at load. Repointed to ./assets/stage.js (which actually provides them).
• Crashing subcommand — removed flavian pipeline ingest (it imported a non-existent runIngest); main exposes ingest via the flavian-ingest bin, untouched.
• Duplicate keys — de-duped pipeline:ingest (root) and flavian-ingest (package).

Regression guard

Adds barrel.test.mjs — it imports the package barrel and asserts every advertised export resolves, failing on any dangling re-export. This is the guard that was missing when the bad merge shipped green.

Test plan

• stage-assets.test.mjs — transform, manifest, end-to-end IDML run aligned with generateTheme() (3)
• barrel.test.mjs — barrel imports cleanly; all exports live (2)
• assets-idml.test.mjs — embedded-asset extraction (2)
• Full @flavian/pipeline suite green (incl. main's ingest / ingest-content / ingest-detect tests) except two pre-existing CRLF-only snapshot tests (stored LF; green on Ubuntu CI)
• InDesign smoke + fixtures (CI steps)
• flavian pipeline ingest now cleanly reports "Unknown pipeline" instead of crashing
• E2E: pnpm pipeline:stage-assets → generate → embedded image staged into theme/assets/
• commitlint passes

Closes #154
Follow-up to #152 (capability issue #117).

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