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/parser.py

@@ -62,7 +62,7 @@ class SxExpr(str):
 # Errors
 # ---------------------------------------------------------------------------
 
-_ESCAPE_MAP = {"n": "\n", "t": "\t", '"': '"', "\\": "\\", "/": "/"}
+_ESCAPE_MAP = {"n": "\n", "t": "\t", "r": "\r", "0": "\0", '"': '"', "\\": "\\", "/": "/"}
 
 
 def _unescape_string(s: str) -> str:
@@ -359,7 +359,9 @@ def serialize(expr: Any, indent: int = 0, pretty: bool = False) -> str:
             expr.replace("\\", "\\\\")
             .replace('"', '\\"')
             .replace("\n", "\\n")
+            .replace("\r", "\\r")
             .replace("\t", "\\t")
+            .replace("\0", "\\0")
             .replace("</script", "<\\/script")
         )
         return f'"{escaped}"'
@@ -392,6 +394,8 @@ def serialize(expr: Any, indent: int = 0, pretty: bool = False) -> str:
             expr.html.replace("\\", "\\\\")
             .replace('"', '\\"')
             .replace("\n", "\\n")
+            .replace("\r", "\\r")
+            .replace("\0", "\\0")
             .replace("</script", "<\\/script")
         )
         return f'(raw! "{escaped}")'