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

test/test_widget_scripts.py

@@ -0,0 +1,48 @@
+"""Guard: every widget JS file must be loaded by base.html or explicitly allowlisted.
+
+Widget files register themselves with LEDMatrixWidgets at load time; a file
+that exists but is never <script>-included silently breaks any plugin whose
+config schema declares that widget (the field renders as an empty container
+that polls the registry forever). base.html's widget list is maintained by
+hand, so this test keeps it honest.
+"""
+from pathlib import Path
+
+PROJECT_ROOT = Path(__file__).resolve().parent.parent
+WIDGETS_DIR = PROJECT_ROOT / 'web_interface' / 'static' / 'v3' / 'js' / 'widgets'
+BASE_HTML = PROJECT_ROOT / 'web_interface' / 'templates' / 'v3' / 'base.html'
+
+# Files that must NOT be script-included, with the reason.
+ALLOWLIST = {
+    # Documentation example (docs/widget-guide.md); registers the name
+    # 'color-picker' and would shadow the real color-picker.js if loaded.
+    'example-color-picker.js',
+}
+
+
+def test_every_widget_script_is_included_in_base_html():
+    base_html = BASE_HTML.read_text(encoding='utf-8')
+    missing = []
+    for js_file in sorted(WIDGETS_DIR.glob('*.js')):
+        if js_file.name in ALLOWLIST:
+            continue
+        if f'v3/js/widgets/{js_file.name}' not in base_html:
+            missing.append(js_file.name)
+    assert not missing, (
+        'Widget files exist but are never <script>-included in base.html '
+        '(plugins declaring these widgets get blank config fields): '
+        + ', '.join(missing)
+        + '. Add a script tag to base.html or add the file to ALLOWLIST '
+        'with a reason.'
+    )
+
+
+def test_allowlisted_widgets_are_not_included():
+    base_html = BASE_HTML.read_text(encoding='utf-8')
+    wrongly_included = [
+        name for name in ALLOWLIST if f'v3/js/widgets/{name}' in base_html
+    ]
+    assert not wrongly_included, (
+        'Allowlisted (must-not-load) widget files are script-included in '
+        'base.html: ' + ', '.join(wrongly_included)
+    )