feat(pipeline): IDML parser and intermediate representation

[PAMulligan]

Closes #62. Foundational sub-issue for the InDesign-to-WordPress pipeline (#61).

Summary

• New @flavian/pipeline workspace package (this PR creates the packages/* workspace; it's the first occupant)
• parseIdml(path) / parseIdmlBuffer(bytes) — unzip via fflate, parse XML via fast-xml-parser, resolve Self/ParentStory cross-references, validate the final shape with zod
• IR covers Document, Spread, Page, Frame (discriminated union of TextFrame + ImageFrame), Story, Style, Swatch, Font, MasterSpread
• Unit normalizer: pt / pc / mm / cm / in / px → px at configurable DPI (default 96)
• Warning collector — missing optional resources, dangling refs, unknown color spaces, empty stories surface on the IR rather than throwing
• CLI: flavian-parse-idml <file.idml> (JSON on stdout, warnings on stderr)
• 20 tests via Node's built-in node:test: happy path + the three malformed cases the issue calls out (missing manifest throws; unknown style refs warn; empty stories give empty run lists) + unit-conversion coverage

Decisions worth flagging

1. Language: issue text says packages/pipeline/src/indesign/ir.ts, but the repo has zero TypeScript today (everything is .mjs ESM + JSDoc + node:test). I used .js + JSDoc + zod so the new package matches the rest of the repo. z.infer<typeof Document> gives any future TS consumer the types for free. Happy to migrate to TS if you'd rather.
2. Fixtures: built programmatically by tests/indesign/helpers/build-idml.js — no binary blobs in git, no InDesign required to maintain them. The epic's "commit .idml fixtures" AC lives on Add InDesign-to-WordPress conversion pipeline #61, not here.

Acceptance criteria (from #62)

• Given a fixture .idml, the parser produces a valid IR object that passes schema validation (zod)
• All swatches, paragraph styles, character styles, and fonts are enumerated in the IR
• Image frames reference resolvable asset paths inside the IDML package (href + embedded flag)
• Unit tests cover happy path + at least three malformed/edge-case IDMLs (missing manifest, unknown style refs, empty stories; also bonus: malformed zip, missing optional resources, unknown color space)
• Parse warnings are collected on the IR and surfaced by the CLI (process.stderr.write for each)

CI

pipeline-tests.yml got a new path filter (packages/pipeline/** and pnpm-workspace.yaml) and a new job that runs pnpm --filter @flavian/pipeline test via pnpm + Node 20. Existing bats jobs unchanged.

Test plan

• pnpm install from a clean clone succeeds (workspace resolution)
• pnpm --filter @flavian/pipeline test reports 20/20 passing
• node packages/pipeline/bin/parse-idml.mjs <real-exported.idml> against a real IDML produces JSON with non-empty swatches, styles, and spreads
• CI Pipeline Tests workflow is green on this PR

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