Add horizontal scroll for diffs with long lines

[backnotprop]

Merge-ready continuation of #1224 by @Karrq.

Opened as a replacement branch because #1224's head branch reports
maintainerCanModify: false, so the prepared merge could not be pushed to the
contributor's own branch. All code in this PR is @Karrq's work — please
attribute the change to them; #1224 should be closed in favour of this PR.

Each file's diff gets its own horizontal scrollbar, pinned to the file header
instead of sitting at the bottom of the file's content where it is off-screen
on any diff taller than the panel.

What this branch adds on top of #1224

• Merged current main and resolved the one conflict in
  packages/review-editor/hooks/usePierreTheme.ts (the two new [data-code]
  scrollbar-suppression rules now sit next to main's .pn-token-hover rule).
• Dropped the PR's bun.lock changes, which reverted main's dependency
  updates. bun install against main's lockfile produces no diff.
• Regenerated packages/core/guide-viewer-manifest.ts for the viewer build.

Verification

• bun test packages/review-editor — 321 pass, 0 fail
• DOM_TESTS=1 bun test --isolate packages/review-editor — 550 pass, 0 fail
• The four isolated real-@pierre/diffs renderer suites — 11 pass, 0 fail
• bun run typecheck — clean
• bun run --cwd apps/review build — succeeds
• check:manifest — in sync
• check:budgets — viewer js 397.9 KB gz (budget 400 KB), css 51.9 KB gz
  (budget 64 KB)

Closes #1566
Closes #1048