feat(keywords): add market insights presentation foundation

[hubab1]

Summary

• add an explicit, collision-safe app/platform/storefront scope for saved market insights
• add a MainActor-isolated, generation-safe model for bounded first-page and cursor pagination reads
• map all seven market evidence states into truthful presentation values, including stale, partial, cached-failure, unavailable, and estimated-difficulty provenance
• preserve rows through cancellation and page failures, with deterministic aggregation and accessibility copy helpers

Dependency and review gate

This draft is stacked on #41 and remains behind the ranking-source and estimated-difficulty semantics gate. It contains no network fan-out or persistence mutation.

The combined item 23 tree remains Category C because the dependent native UI needs maintainer UX review. No live app launch was performed because this stack inherits schema V4.

Validation

• clean foundation worktree: 16/16 focused tests passed
• combined item 22 and item 23 suites: 38/38 passed
• exact integrated tree: 446/446 full tests passed
• signed Debug build and strict codesign verification passed
• Swift 6.2 complete-concurrency build, project plist lint, diff check, and independent review passed
• no P0, P1, or P2 findings remain

Reference

Independent implementation informed by the market-insights direction in WIP PR #8 commit akshaynexus@441b871. Source reference only.