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/async_eval.py

@@ -1333,7 +1333,7 @@ async def _aser(expr: Any, env: dict[str, Any], ctx: RequestContext) -> Any:
                 expanded = _expand_macro(val, expr[1:], env)
                 return await _aser(expanded, env, ctx)
             if isinstance(val, Component):
-                if _expand_components.get() or not val.is_pure:
+                if _expand_components.get() or val.render_target == "server":
                     return await _aser_component(val, expr[1:], env, ctx)
             return await _aser_call(name, expr[1:], env, ctx)