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/jinja_bridge.py

@@ -198,6 +198,10 @@ def register_components(sx_source: str) -> None:
                 all_classes = scan_classes_from_sx(sx_source)
             val.css_classes = set(all_classes)
 
+    # Recompute transitive deps for all components (cheap — just AST walking)
+    from .deps import compute_all_deps
+    compute_all_deps(_COMPONENT_ENV)
+
     _compute_component_hash()
 
 
@@ -322,6 +326,71 @@ def client_components_tag(*names: str) -> str:
     return f'<script type="text/sx" data-components>{source}</script>'
 
 
+def components_for_page(page_sx: str) -> tuple[str, str]:
+    """Return (component_defs_source, page_hash) for a page.
+
+    Scans *page_sx* for component references, computes the transitive
+    closure, and returns only the definitions needed for this page.
+    The hash is computed from the page-specific bundle for caching.
+    """
+    from .deps import components_needed
+    from .parser import serialize
+
+    needed = components_needed(page_sx, _COMPONENT_ENV)
+    if not needed:
+        return "", ""
+
+    # Also include macros — they're needed for client-side expansion
+    parts = []
+    for key, val in _COMPONENT_ENV.items():
+        if isinstance(val, Component):
+            if f"~{val.name}" in needed or key in needed:
+                param_strs = ["&key"] + list(val.params)
+                if val.has_children:
+                    param_strs.extend(["&rest", "children"])
+                params_sx = "(" + " ".join(param_strs) + ")"
+                body_sx = serialize(val.body, pretty=True)
+                parts.append(f"(defcomp ~{val.name} {params_sx} {body_sx})")
+        elif isinstance(val, Macro):
+            # Include macros that are referenced in needed components' bodies
+            # For now, include all macros (they're small and often shared)
+            param_strs = list(val.params)
+            if val.rest_param:
+                param_strs.extend(["&rest", val.rest_param])
+            params_sx = "(" + " ".join(param_strs) + ")"
+            body_sx = serialize(val.body, pretty=True)
+            parts.append(f"(defmacro {val.name} {params_sx} {body_sx})")
+
+    if not parts:
+        return "", ""
+
+    source = "\n".join(parts)
+    digest = hashlib.sha256(source.encode()).hexdigest()[:12]
+    return source, digest
+
+
+def css_classes_for_page(page_sx: str) -> set[str]:
+    """Return CSS classes needed for a page's component bundle + page source.
+
+    Instead of unioning ALL component CSS classes, only includes classes
+    from components the page actually uses.
+    """
+    from .deps import components_needed
+    from .css_registry import scan_classes_from_sx
+
+    needed = components_needed(page_sx, _COMPONENT_ENV)
+    classes: set[str] = set()
+
+    for key, val in _COMPONENT_ENV.items():
+        if isinstance(val, Component):
+            if (f"~{val.name}" in needed or key in needed) and val.css_classes:
+                classes.update(val.css_classes)
+
+    # Page sx is unique per request — scan it
+    classes.update(scan_classes_from_sx(page_sx))
+    return classes
+
+
 def sx_css_all() -> str:
     """Return all CSS rules (preamble + utilities) for Jinja fallback pages."""
     from .css_registry import get_all_css