Cache parsed components for 10x faster startup (2s → 200ms)

- Fix O(n²) postprocessing: compute_all_deps/io_refs/hash were called
  per-file (92x for sx app). Now deferred to single finalize_components()
  call after all files load.
- Add pickle cache in shared/sx/.cache/ keyed by file mtimes+sizes.
  Cache stores fully-processed Component/Island/Macro objects with deps,
  io_refs, and css_classes pre-computed. Closures stripped before pickle,
  rebuilt from global env after restore.
- Smart finalization: cached loads skip deps/io_refs recomputation
  (already in pickle), only recompute component hash.
- Fix ~sx-header → ~layouts/header ref in docs-content.sx

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1,6 +1,7 @@
 __pycache__/
 *.pyc
 *.pyo
+shared/sx/.cache/
 .env
 node_modules/
 *.egg-info/