refactor(desktop): align the design contracts with the code - #239

Outcome

The governance item of the agreed repair order: the design system's machine-readable contracts now
describe what the code does, the shared module the doc omitted is registered, and the loading
contract has real callers.

• layout-spec.json takes the token values it had drifted from: titlebar 48 → 46 (both places),
  normal control 28 → 32, field control 32 → 36. The rest of the file was audited and already
  matched the code (rail 288 with 220/420 clamps, dock 440, spacing 2–32, the 768 content and
  settings measures, the 800/1000/1400 breakpoints).
• A new tests/designContract.test.ts resolves the token aliases and asserts the spec against them,
  so editing one side without the other fails the suite.
• docs/design/system.md registers src/settings/SettingsPrimitives.tsx as the settings-scoped
  composition with its real callers, and adds the settings pages to the PageHeader caller list.
• github/PullRequestsPage.tsx routes its list and detail loading/failure states through the shared
  LoadFeedback, which until now had no product caller while the page hand-rolled its exact shape
  four times.
• Correction to the earlier audit in this thread: ControlChip is not a dead contract — the
  composer and SceneChip call it through its Chip alias, nine call sites in total. The record
  carries the correction.

Change record

• Change: docs/sdlc/changes/2026-09-16-align-design-contracts/intent.md

The linked change directory owns scope, authorization, acceptance, evidence, and residual risk.

Review and release

• Acceptance evidence covers the current diff, including rendered UI where affected: the
  failure state rendered through [data-slot="load-feedback"][data-state="error"] (role="alert"
  + Retry) and the loading state through [data-state="loading"]/role="status" by temporarily
  forcing the UI Lab fixture, which was restored byte-identically; the record's AC-1..AC-5 map to
  current PASS evidence.
• Required checks passed on this base: bun run lint, bunx tsc --noEmit (inside
  build:renderer), bun test (963 pass / 3 skip / 0 fail), bun run build:renderer,
  bun script/verify/sdlc.ts --worktree, bun script/verify/docs.ts, git diff --check.
• Merge authorization: pending (this PR was requested; merge is not authorized)
• Release authorization: not requested