Document SX rendering pipeline, add missing sx_docs mount, loud error on missing component

- CLAUDE.md: add SX rendering pipeline overview, service sx/ vs sxc/
  convention, dev container mount convention
- docker-compose.dev.yml: add missing ./sx/sx:/app/sx bind mount for
  sx_docs (root cause of "Unknown component: ~sx-layout-full")
- async_eval.py: add evaluation modes table to module docstring; log
  error when async_eval_slot_to_sx can't find a component instead of
  silently falling through to client-side serialization
- helpers.py: remove debug logging from render_to_sx_with_env

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

shared/sx/async_eval.py

@@ -10,6 +10,28 @@ control flow (``if``, ``let``, ``map``, ``when``).  The sync
 collect-then-substitute resolver can't handle data dependencies between
 I/O results and control flow, so handlers need inline async evaluation.
 
+Evaluation modes
+~~~~~~~~~~~~~~~~
+
+The same component AST can be evaluated in different modes depending on
+where the rendering boundary is drawn (server vs client).  Five modes
+exist across the codebase:
+
+  Function              Expands components?  Output          Used for
+  --------------------  -------------------  --------------  ----------------------------
+  _eval (sync)          Yes                  Python values   register_components, Jinja sx()
+  _arender (async)      Yes                  HTML            render_to_html
+  _aser (async)         No — serializes      SX wire format  render_to_sx
+  _aser_component       Yes, one level       SX wire format  render_to_sx_with_env (layouts)
+  sx.js renderDOM       Yes                  DOM nodes       Client-side
+
+_aser deliberately does NOT expand ~component calls — it serializes them
+as SX wire format so the client can render them.  But layout components
+(used by render_to_sx_with_env) need server-side expansion because they
+depend on Python context (auth state, fragments, etc.).  That's what
+_aser_component / async_eval_slot_to_sx provides: expand the top-level
+component body server-side, then serialize its children for the client.
+
 Usage::
 
     from shared.sx.async_eval import async_render
@@ -1021,6 +1043,16 @@ async def async_eval_slot_to_sx(
                 if result is None or result is NIL:
                     return ""
                 return serialize(result)
+            else:
+                import logging
+                logging.getLogger("sx.eval").error(
+                    "async_eval_slot_to_sx: component %s not found in env "
+                    "(will fall through to _aser and serialize unexpanded — "
+                    "client will see 'Unknown component'). "
+                    "Check that the .sx file is loaded and the service's sx/ "
+                    "directory is bind-mounted in docker-compose.dev.yml.",
+                    head.name,
+                )
     # Fall back to normal async_eval_to_sx
     result = await _aser(expr, env, ctx)
     if isinstance(result, SxExpr):