Add post-closeout guard and monitoring for fullscreen rules.

Ensures no regression after Level 23 closeout.

PR: LEVEL 23.11 POST CLOSEOUT GUARD

Author: DavidQ

docs/dev/reports/post_closeout_monitoring.txt

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+Post Closeout Monitoring
+
+- Rule remains: fullscreen only in sample 0713
+- Any future addition outside 0713 = violation
+- Validation automation must be rerun on PRs touching samples
+- Required automation reuse for future PRs that touch `samples/`:
+  - `rg -n --glob "samples/**" "requestFullscreen|webkitRequestFullscreen|engine\\.fullscreen|fullscreenPreferred|settings-fullscreen"`
+  - `node --input-type=module -e "import('./tests/samples/FullscreenRuleEnforcement.test.mjs').then(m=>m.run())"`
+  - `node --input-type=module -e "import('./tests/samples/FullscreenAbility0713ViewportFit.test.mjs').then(m=>m.run())"`

docs/dev/validation_checklist.txt

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+Validation Checklist
+
+[x] Guard rules persist
+[x] Monitoring defined
+[x] No regression introduced
+[x] Fullscreen scan command documented for reuse
+[x] `FullscreenRuleEnforcement` test reuse documented
+[x] `FullscreenAbility0713ViewportFit` test reuse documented

docs/pr/BUILD_PR_LEVEL_23_11_POST_CLOSEOUT_GUARD_AND_MONITORING.md

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+# BUILD PR LEVEL 23.11
+## Post-Closeout Guard and Monitoring
+
+### Purpose
+Ensure fullscreen rule remains permanently enforced after closeout.
+
+### Scope
+- Reinforce guard rules
+- Add ongoing monitoring expectations
+- Prevent regression
+- Reuse existing validation automation (do not create duplicate checks)
+
+### Required Reuse On Future PRs Touching `samples/`
+- Run fullscreen scan:
+  - `rg -n --glob "samples/**" "requestFullscreen|webkitRequestFullscreen|engine\\.fullscreen|fullscreenPreferred|settings-fullscreen"`
+- Run fullscreen guard tests:
+  - `tests/samples/FullscreenRuleEnforcement.test.mjs`
+  - `tests/samples/FullscreenAbility0713ViewportFit.test.mjs`
+
+### Acceptance
+- Fullscreen remains restricted to sample 0713
+- Any regression is detectable
+- No engine changes