test: add regression guards for the bug classes fixed in this PR

Three lightweight static checks, all enrolled in CI's unit-test
allowlist along with the new web-cache test:

- test_template_targets.py: every literal render_template() target must
  exist (would have caught the weather/stocks partial 500s at commit
  time).
- test_widget_scripts.py: every widget JS file must be script-included
  in base.html or explicitly allowlisted with a reason (would have
  caught the unloaded time-picker/file-upload-single/plugin-file-manager
  widgets), and allowlisted files must NOT be included (prevents the
  example widget from shadowing the real color-picker).
- test_doc_links.py: relative markdown links in active docs must
  resolve (docs/archive/ exempt).

Each guard was verified to fail against the pre-PR tree and pass now.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SXb4mKcAkVaxkeTb3YnAdr

.github/workflows/test.yml

@@ -80,4 +80,8 @@ jobs:
             test/test_version_consistency.py \
             test/test_plugin_compatibility_gate.py \
             test/test_install_preserves_existing.py \
-            test/test_core_owned_config_keys.py
+            test/test_core_owned_config_keys.py \
+            test/test_template_targets.py \
+            test/test_widget_scripts.py \
+            test/test_doc_links.py \
+            test/web_interface/test_cache.py