feat: support github's react-based diff ui

[hashiiiii]

Summary

Support GitHub's new React-based "Files changed" UI (login-gated rollout, /pull/N/changes) in the Chrome extension: Unity-file detection, the per-file toggle, semantic-view injection, and the global toggle bar now work on both the classic and the React layouts.

Motivation

The extension only matched the classic DOM (.file-header[data-path], .js-file-content); on the React UI those selectors match nothing and the extension silently does nothing. The React UI has been GitHub's default for logged-in users since late 2025, so the product's primary channel was at risk.

Closes #159

Changes

• parsePrUrl accepts /pull/N/changes and /pull/N/changes/<sha>..<sha> (a single sha under /changes/ is the commit view and stays out of scope), mirroring github-url-detection
• detect.ts: FileEntry is now a per-layout adapter (attachHost / setRawHidden / collapsed / globalAnchor); the classic and React scanners always both run, and the non-matching one yields [] harmlessly — no layout probe needed
• React scanner anchors on div[role="region"][id^="diff-"] plus hashed CSS-module class prefixes; the path is parsed from header text (LRM marks stripped, sr-only rename form preferred) because the React UI has no path attribute anywhere
• content/index.ts: mounting goes through the adapter; a display-only, fetch-free sync() re-asserts hide/show on every scan because the React UI virtualizes the file list (TanStack Virtual) and recreates DOM nodes continuously; disconnected appliers are pruned every scan instead of only on PR change
• Collapse is read from two independent signals: the chevron octicon swap and the DiffFileHeader-module__collapsed header class
• e2e: new fixture pr-files-react.html + react.spec.ts covering detection/render, global-bar placement outside the virtualized list, collapse chevron + body remount re-hide, and a full virtualization remount inheriting the semantic default

React-UI DOM structure was cross-checked against refined-github, TombWater/github-codeowners, sudo-vaibhav/pr-diff-plus, dkareh/bookmarklets, DIO0550/github-html-preview-ext, a live-captured fixture (elad12390/group-gh-pr-files), and GitHub's own shipped component chunk (confirms role="region", id="diff-${pathDigest}", header wrapper as first region child, and the CSS-module class map).

Testing

• pnpm test — 197 passed (17 files)
• pnpm typecheck — clean
• pnpm lint — exit 0
• pnpm e2e — 14 passed (10 existing classic specs unchanged + 4 new react specs)

Live verification on the real React UI (performed 2026-07-16)

Verified on https://github.com/hashiiiii/unity-yaml-playground/pull/2/changes in a logged-in browser session, by injecting the built dist/content.js with the same chrome.* stub the e2e suite uses (canned semantic-diff response; the background fetch/wasm path is covered separately by the existing unit + classic e2e layers):

• Scanner matched all 21 file regions; toggles attached to exactly the 19 UnityYAML files (both .meta files correctly excluded, including a renamed one)
• Path extraction verified against the live DOM, including the rename case (sr-only "renamed to" form returns the new path)
• Global bar rendered once, immediately before [data-testid="progressive-diffs-list"]
• Semantic toggle: shadow-root view rendered, real React diff body hidden
• Real collapse round-trip: chevron click unmounts the body (region children 2 → 1) and the semantic host hides; re-expand remounts a fresh body node which the per-scan sync() re-hides while the semantic view returns
• Live DOM matched the fixture assumptions exactly (Diff-module__diffHeaderWrapper as first region child, chevron icon swap, DiffFileHeader-module__collapsed class stamped when collapsed)

Manual verification (for reviewers)

1. Sign in to github.com with the new Files changed experience enabled (or click "Try the new experience" on a PR)
2. Open a PR with UnityYAML changes (e.g. feat: mutate fixture corpus for diff demo unity-yaml-playground#2) and go to the Files changed tab (/changes)
3. Confirm: the PrefabLens global bar sits above the diff list, each Unity file header has the Raw/Semantic toggle, Semantic replaces the raw diff, collapsing a file hides the semantic view, and scrolling far away and back preserves the chosen view