feat: unify gap analysis with verify results and source provenance

Single source of truth for gap page: verification status from
verify.py (verified/untested/missing/mismatch), file provenance
from cross_reference (bios/data/large_file/missing).

cross_reference.py: _find_in_repo -> _resolve_source returning
source category, stop skipping storage: release/large_file,
add by_path_suffix lookup, all_declared param for global check.

generate_site.py: gap page now shows verification by platform,
18 hash mismatches, and core complement with provenance breakdown.

mkdocs.yml

@@ -20,6 +20,8 @@ theme:
       icon: material/brightness-4
       name: Switch to auto
   font: false
+  icon:
+    logo: material/chip
   features:
   - navigation.tabs
   - navigation.sections
@@ -29,6 +31,8 @@ theme:
   - search.highlight
   - content.tabs.link
   - toc.follow
+extra_css:
+- stylesheets/extra.css
 markdown_extensions:
 - tables
 - admonition