feat: restructure routed docs and release surface

[xuewill]

Summary

• split the documentation app into landing, component index, component detail, and release routes
• rebuild the shared header, footer, search, sidebar, and component specimen surfaces from the approved UI prototypes
• add nested-route deployment fallback, direct reload/history coverage, and the landing reveal fix
• expand the public WeBaseIcon Lucide vocabulary with a patch changeset
• retain the release-candidate screen-reader audit record already prepared on this branch

Validation

• npm audit --audit-level=high (0 vulnerabilities)
• npm run check
• npm run check:changeset
• npm run check:consumer
• npm run test:browser (102 passed across Chromium, Firefox, and WebKit)
• npm run test:visual (47 passed)
• git diff --check

Reviewed size change

The expanded static icon map raises WeBaseIcon source gzip from 1,052 B to 1,357 B and the consumer JS gzip baseline from 21,156 B to 22,262 B. This deliberate public API cost is documented in docs/PERFORMANCE.md and the package-size baseline is refreshed.

Release gate

This PR remains draft. docs/SCREEN_READER_AUDIT.md still has pending human records for VoiceOver/Safari and NVDA/Firefox-or-Chrome. Per docs/RELEASE_RUNBOOK.md, do not merge this PR or the stable Changesets release PR until both required environments have direct passing records.