Fix CI: regenerate stale generated files; remove unused import

After editing example markdown sources (strings, testing, logging,
datetime, networking, subprocesses, threads-and-processes,
virtual-environments) the embedded sources data and the HTML cache
fingerprint had drifted. `make check-generated` caught this in CI.

  src/example_sources_data.py — regenerated by
    scripts/embed_example_sources.py
  src/asset_manifest.py — HTML_CACHE_VERSION updated by
    scripts/fingerprint_assets.py (depends on the embedded data)
  scripts/build_marginalia.py — Canvas was imported but unused
    after the gestalt-uses-production refactor; ruff caught it

58 tests pass; lint clean; quality-checks clean.

Author: claude

scripts/build_marginalia.py

@@ -24,7 +24,7 @@
 sys.path.insert(0, str(ROOT / "src"))
 
 from marginalia import ATTACHMENTS, FIGURES, SCORES  # noqa: E402  (sys.path set above)
-from marginalia_grammar import Canvas, Card  # noqa: E402
+from marginalia_grammar import Card  # noqa: E402
 from example_loader import load_examples  # noqa: E402
 
 OUT = ROOT / "public" / "prototyping" / "marginalia-gestalt.html"

src/asset_manifest.py

@@ -1,3 +1,3 @@
 # Generated by scripts/fingerprint_assets.py. Do not edit by hand.
 ASSET_PATHS = {'SITE_CSS': '/site.1452cc5609f2.css', 'SYNTAX_JS': '/syntax-highlight.3b6c7f730d46.js', 'EDITOR_JS': '/editor.dd81f5171b14.js'}
-HTML_CACHE_VERSION = '5004d5df5c18'
+HTML_CACHE_VERSION = '2ef350ca9050'