[FIR-294] Flydocs visual-parity gate: theme port, parity page and VR pipeline

[mfg1986]

What

Implements the FF-CAT-20 phase-2 gate (FIR-294): no Flydocs screen migrates until visual parity of the ported theme is validated.

• Flydocs theme over --ff-*: apps/playground/src/app/themes/_flydocs-theme.scss (+ dark), a playground-only fixture scoped under :root[data-brand="flydocs"]. Token-by-token equivalence (72 real tokens: 49 direct, 9 derived, 14 deliberately left un-overridden) documented in docs/flydocs-theme-token-equivalence.md.
• /parity QA page: one section per migration wave (1-5) rendering the dominant real prop combinations from the usage inventory, plus BrandThemeService and a brand toggle in the playground shell.
• Visual regression: Playwright screenshot-diff of the 5 wave sections x light/dark plus the ff-input rest/hover/focus/error/disabled states (the focus-ring moved to the field wrapper in FIR-289 and is explicitly covered). Chromium-only baselines, dedicated port 4300, deterministic rendering. New e2e CI job with a one-click seed_vr_baselines dispatch input for the linux baseline bootstrap (docs/visual-regression.md).
• Diff report: docs/flydocs-parity-report.md — 13/15 components with no perceptible diff at 100% zoom; 2 flagged deltas (neutral chip cool-vs-warm, focus-ring solid-vs-halo) and 2 gaps (no ff-textarea/ff-slider) left for their own issues per the gate's contract.

Why

Gate for phase 2 of the Flydocs migration (ETAPA 2 of the component-system roadmap). Parity reference is the rendered product, never the Figma palettes.

How to verify

• pnpm nx e2e playground-e2e — 15/15 against the committed darwin baselines.
• pnpm nx serve playground → open /parity, toggle "Flydocs theme" / "Dark mode".
• Post-merge/pre-review note: the CI e2e job stays red until the linux baselines are seeded — run the CI workflow manually with seed_vr_baselines checked, download the playground-e2e-vr-baselines artifact, review and commit the -linux.png set (procedure in docs/visual-regression.md).

Traceability

Board ticket without an owning PASO (pre-methodology playground scope): ticket key carried in branch and commit subjects.