docs(bf-5kk): standardize on ai-code-battle.pages.dev as canonical public domain

- Update replay-schema-v1.json  to pages.dev
- Update robots.txt sitemap URL to pages.dev
- Update test-match-list.html thumbnail URLs to pages.dev/r2/
- Add decision note documenting standardization

All user-facing absolute URLs now use the working pages.dev origin.
The aicodebattle.com domain is NXDOMAIN and was never registered.
R2 data is served through Pages Functions (/r2/*) eliminating the
need for a separate b2.aicodebattle.com CDN host.

Co-Authored-By: Claude <noreply@anthropic.com>

docs/notes/canonical-public-domain.md

@@ -0,0 +1,72 @@
+---
+name: canonical-public-domain
+description: Standardized on ai-code-battle.pages.dev as canonical public domain
+metadata:
+  type: project
+---
+
+# Canonical Public Domain Decision
+
+## Decision
+
+**Standardized on `ai-code-battle.pages.dev` as the canonical public domain.**
+
+## Background
+
+The domain `aicodebattle.com` is NXDOMAIN (not registered). The site is deployed and functional at `ai-code-battle.pages.dev`. References to `aicodebattle.com` and `b2.aicodebattle.com` existed in documentation and test files.
+
+## Rationale
+
+- Registering `aicodebattle.com` requires human action and payment
+- Cloudflare Pages provides a production-ready public URL at `ai-code-battle.pages.dev`
+- R2 storage is now served through Pages Functions (`/r2/*` path), eliminating the need for a separate `b2.aicodebattle.com` CDN host
+- Standardizing on pages.dev ensures all user-facing URLs work immediately
+
+## Changes Made
+
+### Updated Files
+
+1. **web/public/replay-schema-v1.json**
+   - Updated `$id` field from `https://aicodebattle.com/replay-schema-v1.json` to `https://ai-code-battle.pages.dev/replay-schema-v1.json`
+
+2. **web/public/robots.txt**
+   - Updated sitemap URL from `https://aicodebattle.com/sitemap.xml` to `https://ai-code-battle.pages.dev/sitemap.xml`
+
+3. **web/public/test-match-list.html**
+   - Updated thumbnail URL from `https://b2.aicodebattle.com/thumbnails/...` to `https://ai-code-battle.pages.dev/r2/thumbnails/...`
+
+### Files Already Correct
+
+- **web/src/og-tags.ts** - Already uses `https://ai-code-battle.pages.dev`
+- **web/src/pages/playlists.ts** - Uses dynamic `window.location.origin` for embed URLs
+- **web/src/pages/clip-maker.ts** - Twitter share uses external domain (correct)
+- **web/index.html** - OG tags already use pages.dev
+- **web/embed.html** - OG tags already use pages.dev
+
+## URL Pattern
+
+All user-facing absolute URLs now follow the pattern:
+
+```
+https://ai-code-battle.pages.dev/<path>
+```
+
+Dynamic URLs use `window.location.origin` which resolves to pages.dev in production.
+
+## Replay and Data URLs
+
+Replays and match data are served through Cloudflare Pages Functions:
+
+- Replays: `https://ai-code-battle.pages.dev/r2/replays/{match_id}.json.gz`
+- Match metadata: `https://ai-code-battle.pages.dev/r2/matches/{match_id}.json`
+- Thumbnails: `https://ai-code-battle.pages.dev/r2/thumbnails/{match_id}.png`
+- Bot cards: `https://ai-code-battle.pages.dev/r2/cards/{bot_id}.png`
+- Evolution status: `https://ai-code-battle.pages.dev/r2/evolution/live.json`
+
+## Future Consideration
+
+If `aicodebattle.com` is registered later, it can be added as a custom domain in Cloudflare Pages. The internal URLs will remain valid as pages.dev will continue to work as the origin.
+
+## Verified
+
+All user-facing absolute URLs emitted by the web application resolve correctly in a browser.

web/public/replay-schema-v1.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://json-schema.org/draft/2020-12/schema",
-  "$id": "https://aicodebattle.com/replay-schema-v1.json",
+  "$id": "https://ai-code-battle.pages.dev/replay-schema-v1.json",
   "title": "AI Code Battle Replay Format",
   "description": "Complete replay format for AI Code Battle matches. Documents the full game state for each turn, enabling third-party tooling and replay viewers.",
   "type": "object",

web/public/robots.txt

@@ -1,4 +1,4 @@
 User-agent: *
 Allow: /
 
-Sitemap: https://aicodebattle.com/sitemap.xml
+Sitemap: https://ai-code-battle.pages.dev/sitemap.xml

web/public/test-match-list.html

@@ -228,7 +228,7 @@
 
         if (matchData.matches && matchData.matches.length > 0) {
           const firstMatch = matchData.matches[0];
-          const thumbnailUrl = `https://b2.aicodebattle.com/thumbnails/${firstMatch.id}.png`;
+          const thumbnailUrl = `https://ai-code-battle.pages.dev/r2/thumbnails/${firstMatch.id}.png`;
 
           try {
             const thumbResponse = await fetch(thumbnailUrl, { method: 'HEAD' });