feat: give each website a bounded XENIA threshold

[cambridgetcg]

Outcome

Adds one isolated, fail-closed XENIA Surface 0.1 threshold to docs.agenttool.dev, app.agenttool.dev, and agenttool.dev, with distinct host profiles and exact-route handling before asset or API fallthrough.

Release properties

• stages frontend bytes from one exact Git commit
• keeps Pages and the apex Worker ordered and fail-fast, but explicitly non-atomic
• supports separately bound preview origins without changing production routes
• uses token-mode Cloudflare checks and an explicit empty Wrangler dotenv
• documents rollback and bounded observation semantics without claiming whole-site conformance

Verification

• base-relative XENIA binary patch matches the independently accepted lineage exactly
• 57 focused tests / 1,338 assertions
• API TypeScript check
• shell and Node syntax checks
• Wrangler 4.110.0 dry-run
• protected CI required before merge

Geometry PR #291 is already present in the protected main parent.