docs: add Sparkle appcast dual-host support documentation

- Add architecture overview for dual-host URL routing (freno.me and *.freno.me)
- Document Vercel JSON rewrite strategy for subdomain pass-through
- Include task completion summary with verification results
- Add SUFeedURL migration guide for new native builds
- Add shell script for verifying dual-host endpoint functionality

docs/sparkle-sufeedurl-migration.md

@@ -0,0 +1,58 @@
+# SUFeedURL Migration for New Native Builds
+
+This document tracks the `SUFeedURL` change needed in the Gaze and InputHalo native Swift repos to use subdomain-based feed URLs.
+
+## Status
+
+- [ ] Gaze: Update `SUFeedURL` in Info.plist (~/Code/Gaze/)
+- [ ] InputHalo: Update `SUFeedURL` in Info.plist (~/Code/InputHalo/)
+
+## SUFeedURL Changes
+
+### Gaze
+
+**File:** `~/Code/Gaze/` — Info.plist
+
+**Current (Legacy):**
+```
+https://freno.me/api/Gaze/appcast.xml
+```
+
+**New (Subdomain):**
+```
+https://gaze.freno.me/api/Gaze/appcast.xml
+```
+
+### InputHalo
+
+**File:** `~/Code/InputHalo/` — Info.plist
+
+**Current (Legacy):**
+```
+https://freno.me/api/InputHalo/appcast.xml
+```
+
+**New (Subdomain):**
+```
+https://inputhalo.freno.me/api/InputHalo/appcast.xml
+```
+
+## Migration Notes
+
+1. **Keep legacy URL working:** Old builds continue to work via the `/api/*` pass-through rewrite on Vercel
+2. **No appcast regeneration needed:** The same S3 appcast files serve both URLs
+3. **EdDSA signatures are host-independent:** No signature changes needed
+4. **Test before release:** Verify Sparkle detects updates with the new subdomain URL in a dev build
+
+## Verification Steps
+
+1. Set `SUFeedURL` in Info.plist to new subdomain URL
+2. Build the app
+3. In the app: "Check for Updates..." → Should find the latest version
+4. Verify the download completes successfully
+5. Verify the EdDSA signature verification passes
+
+## Related
+
+- [Dual-Host Support Documentation](./sparkle-dual-host-support.md)
+- [Verification Script](../scripts/verify-sparkle-dual-host.sh)