perf(web): reduce design-system preview request churn #7079

Problem

Browsing bundled design systems from the home composer can feel sluggish on a slower daemon or filesystem. The picker eagerly resolves a full design-system detail for transient hover/focus states, and overlapping consumers can repeat the same read.

With a local production build and 300 ms simulated latency on GET /api/design-systems/:id, rapidly scanning the first eight official presets produced eight detail HTTP attempts and repeatedly replaced the preview pane with a loading state.

Reproduction

1. Run pnpm tools-dev run web --prod --no-env-file.
2. Open the design-system picker from the home composer.
3. Add 300 ms latency to GET /api/design-systems/:id.
4. Move quickly through the first eight official presets without selecting one.

Expected behavior

• Transient hover/focus states should not start a detail read.
• A stable preview target should load once, even when multiple consumers request it concurrently.
• Bundled preset details should be reusable during the browsing burst.
• Editable systems must still show fresh content immediately after a write.
• Reading one bundled detail should not require parsing the full design-system catalog.

Proposed scope

• Add a short shared hover/focus intent before activating the preview.
• Coalesce overlapping detail, preview, and showcase reads.
• Cache immutable bundled details briefly while keeping editable-system reads fresh.
• Invalidate and fence in-flight detail reads after successful mutations.
• Resolve bundled detail summaries directly with canonical, case-sensitive IDs.

Verification targets

• A red-on-main/green-on-branch test for hover/focus intent.
• Registry tests for coalescing, abort behavior, cache lifetime, write invalidation, and stale in-flight fencing.
• Daemon tests proving single-ID reads avoid a catalog scan and reject non-canonical casing.
• Local before/after measurement under the same simulated latency.