Phase 7a: affinity annotations + fix parser escape sequences

Add :affinity :client/:server/:auto annotations to defcomp, with
render-target function combining affinity + IO analysis. Includes
spec (eval.sx, deps.sx), tests, Python evaluator, and demo page.

Fix critical bug: Python SX parser _ESCAPE_MAP was missing \r and \0,
causing bootstrapped JS parser to treat 'r' as whitespace — breaking
all client-side SX parsing. Also add \0 to JS string emitter and
fix serializer round-tripping for \r and \0.

Reserved word escaping: bootstrappers now auto-append _ to identifiers
colliding with JS/Python reserved words (e.g. default → default_,
final → final_), so the spec never needs to avoid host language keywords.

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

shared/sx/evaluator.py

@@ -565,7 +565,7 @@ def _sf_defkeyframes(expr: list, env: dict) -> Any:
 
 
 def _sf_defcomp(expr: list, env: dict) -> Component:
-    """``(defcomp ~name (&key param1 param2 &rest children) body)``"""
+    """``(defcomp ~name (&key ...) [:affinity :client|:server] body)``"""
     if len(expr) < 4:
         raise EvalError("defcomp requires name, params, and body")
     name_sym = expr[1]
@@ -593,21 +593,38 @@ def _sf_defcomp(expr: list, env: dict) -> Component:
                     params.append(p.name)
             else:
                 params.append(p.name)
-        # Skip children param name after &rest
         elif isinstance(p, str):
             params.append(p)
 
+    # Body is always last element; keyword annotations between params and body
+    body = expr[-1]
+    affinity = _defcomp_kwarg(expr, "affinity", "auto")
+
     comp = Component(
         name=comp_name,
         params=params,
         has_children=has_children,
-        body=expr[3],
+        body=body,
         closure=dict(env),
+        affinity=affinity,
     )
     env[name_sym.name] = comp
     return comp
 
 
+def _defcomp_kwarg(expr: list, key: str, default: str) -> str:
+    """Extract a keyword annotation from defcomp, e.g. :affinity :client."""
+    # Scan from index 3 to second-to-last for :key value pairs
+    for i in range(3, len(expr) - 1):
+        item = expr[i]
+        if isinstance(item, Keyword) and item.name == key:
+            val = expr[i + 1]
+            if isinstance(val, Keyword):
+                return val.name
+            return str(val)
+    return default
+
+
 def _sf_begin(expr: list, env: dict) -> Any:
     if len(expr) < 2:
         return NIL