fix(capture): declare prefers-reduced-motion before navigation

[BenSheridanEdwards]

Why does this feature exist?

Validating 4.6.0 against a real consumer surfaced a determinism gap in the capture itself: a JS-driven entrance animation (framer-motion, blur(10px) → blur(0px)) raced the settle, and the self-check correctly failed the surface as "non-deterministic — 1 computed-style difference(s) between two captures of the same commit" — on every cold capture, across multiple PRs. FREEZE_CSS cannot reach this class of motion: those libraries write inline styles from rAF loops that no stylesheet overrides. But they all honour prefers-reduced-motion, and they read it at mount — the capture just never declared it. Out-of-the-box determinism shouldn't depend on each consumer wiring reduced-motion emulation into their own Playwright config.

What changed?

1. src/runner.ts — captureSurface emulates reducedMotion: 'reduce' before go(). Declared before the first navigation because mount-time is when JS animation libraries read the media query; page.emulateMedia persists across every later go()/reset in the same flow (state resets, popup captures, the determinism self-check's re-navigation).
2. src/capture-url.ts — captureUrlToDir emulates it before its first goto, covering the ad-hoc URL-capture path and the crawl.
3. src/capture.ts — captureStyleMap declares it defensively for direct library users, beside the existing FREEZE_CSS injection, with a comment explaining why both exist (CSS-declared motion vs rAF-driven inline styles).

Consumers whose components branch on reduced motion will see a one-time baseline shift to the motionless final state — the deterministic state a style gate should certify; the CHANGELOG entry says so explicitly.

Behavioural Proof (with video and screenshots)

Not applicable — capture-determinism logic with no rendered surface of its own. The new e2e fixture pins the mechanism executably: a page whose mount script colours an element by the mount-time value of prefers-reduced-motion, captured through captureUrlToDir. It passes with the pre-navigation emulation and fails without it (verified both ways locally). The first fixture attempt asserted on the mid-flight race itself and could not bite (the settle's volatile-region detection masked it) — the committed fixture pins the declared media state instead, deterministic in both directions.

Verification Summary

• Full unit suite: npm test — 587/587 pass.
• Full e2e suite: npx playwright test — 115/115 pass, including the new reduced-motion fixture and all existing capture/crawl/dogfood specs (no baseline pins shifted).
• Negative control: reverting the three emulation call sites and rerunning npx playwright test -g "reduced-motion" fails as expected.
• Pre-commit gate green: tsc build + typecheck, eslint, prettier, gitleaks, fallow, commitlint.

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