Implement isomorphic Phase 1: per-page component bundling

Add component dependency analyzer (shared/sx/deps.py) that walks
component AST bodies to compute transitive dependency sets. sx_page()
and sx_response() now send only the components each page needs instead
of the entire registry.

Changes:
- New: shared/sx/deps.py — transitive_deps(), components_needed(),
  scan_components_from_sx(), compute_all_deps()
- shared/sx/types.py — Add deps: set[str] field to Component
- shared/sx/jinja_bridge.py — Compute deps on registration, add
  components_for_page() and css_classes_for_page()
- shared/sx/helpers.py — sx_page() uses per-page bundle + hash,
  sx_response() passes source to components_for_request() for
  page-scoped component diffing
- New: shared/sx/tests/test_deps.py — 15 tests covering AST scanning,
  transitive deps, circular refs, per-page bundling

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

shared/sx/types.py

@@ -167,6 +167,7 @@ class Component:
     body: Any                 # unevaluated s-expression body
     closure: dict[str, Any] = field(default_factory=dict)
     css_classes: set[str] = field(default_factory=set)  # pre-scanned :class values
+    deps: set[str] = field(default_factory=set)        # transitive component deps (~names)
 
     def __repr__(self):
         return f"<Component ~{self.name}({', '.join(self.params)})>"