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_doc_links.py

@@ -0,0 +1,40 @@
+"""Guard: relative markdown links in active docs must resolve.
+
+Scans repo-root *.md and docs/ (excluding docs/archive/, which is allowed
+to rot). External URLs, mailto links, and pure anchors are skipped, as are
+links inside fenced code blocks.
+"""
+import re
+from pathlib import Path
+
+PROJECT_ROOT = Path(__file__).resolve().parent.parent
+LINK_RE = re.compile(r'\[[^\]]*\]\(([^)\s]+)\)')
+FENCE_RE = re.compile(r'^(```|~~~)')
+
+
+def _md_files():
+    yield from PROJECT_ROOT.glob('*.md')
+    for path in PROJECT_ROOT.glob('docs/**/*.md'):
+        if 'archive' not in path.parts:
+            yield path
+
+
+def test_relative_markdown_links_resolve():
+    broken = []
+    for md in _md_files():
+        in_fence = False
+        for lineno, line in enumerate(md.read_text(encoding='utf-8').splitlines(), 1):
+            if FENCE_RE.match(line.strip()):
+                in_fence = not in_fence
+                continue
+            if in_fence:
+                continue
+            for target in LINK_RE.findall(line):
+                if target.startswith(('http://', 'https://', 'mailto:', '#')):
+                    continue
+                resolved = (md.parent / target.split('#')[0]).resolve()
+                if not resolved.exists():
+                    broken.append(
+                        f'{md.relative_to(PROJECT_ROOT)}:{lineno} -> {target}'
+                    )
+    assert not broken, 'Broken relative markdown links:\n' + '\n'.join(broken)