feat(site): publish LHTB five-arm research brief

[shangzh0]

Summary

• publish a bilingual /benchmarks/lhtb/ brief comparing five execution mechanisms across all 46 LHTB tasks
• ship the public-safe aggregate, mechanism insights, representative gains/losses, and searchable score matrix
• register the route through the full Frontstage Pages export contract so static hosting produces site/benchmarks/lhtb/index.html

Review feedback addressed

This supersedes #4703 and directly addresses its requested changes:

• add benchmarks/lhtb to copyPublicSiteRoutes()
• add lhtb_brief_entry, content_sources.lhtb_brief, generated brief URL, and bundle README reference
• assert route content, manifest fields, README publication, and homepage discovery in Frontstage smokes
• add the route to the design-baseline destination list
• list the study in the top-level and benchmark research indexes
• add an LHTB publication-scope test for 46 unique tasks, five-arm row/schema parity, recomputed means/pass counts, and bilingual arm/mechanism keys

Evidence boundary

• one effective trial per task-arm cell; no repeated-seed causal claim
• New Heartbeat changes multiple runtime dimensions together
• public aggregate excludes raw trajectories, local paths, gateway details, credentials, and verifier artifacts
• LHTB reward and the >= 0.95 solved threshold remain benchmark-native

Validation

• npm run build in apps/presentation/site
• python3 -m pytest benchmark/tests/test_publication_scope.py -q (4 passed)
• node examples/dashboard-frontstage-design-baseline-smoke.mjs
• node examples/frontstage-share-bundle-smoke.mjs on current upstream + this PR
• root-base static export served over HTTP: /benchmarks/lhtb/?lang=zh returns 200, renders 46 rows on 1440px and 390px viewports, has no console errors, and has no document-level horizontal overflow

[shangzh0]

Superseded by #4709. #4709 is rebased on current main and contains only the remaining LHTB Pages publication contract and regression coverage, eliminating this PRs stale-fork conflicts.