feat: add Base2026 Signal Lab analytics

[offflinerpsy]

Summary

• add deterministic signal_lab.json generator from public Base2026 JSON/JSONL
• replace /knowledge/analytics.html with compact Signal Lab UI and page-scoped analytics.js
• vendor Apache ECharts only for the analytics page and document the third-party notice
• wire packaging/API/data-dictionary/publication audit for signal_lab.json

Test Plan

• node --check web/static/analytics.js
• python3 -m py_compile scripts/generate-base2026-signal-lab.py scripts/generate-public-pages.py scripts/audit-publication-boundary.py
• python3 scripts/generate-base2026-signal-lab.py --data public-data/tiktok --out public-data/tiktok/signal_lab.json
• python3 scripts/check-public-export-policy.py public-data/tiktok
• python3 scripts/validate-public-release-contract.py --export-dir public-data/tiktok --baseline-export-dir public-data/tiktok --enforce-count-floor --max-violations 20
• python3 scripts/audit-publication-boundary.py
• git diff --check
• production-like local visual smoke at 1440x1000, 1159x900, 768x1024, 390x844, 320x740

Notes

• no raw captions/ASR/media/private QA committed
• no generated public-data committed
• deploy/reindex intentionally handled separately from this PR