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>

sx/sx/nav-data.sx

@@ -125,7 +125,8 @@
   (dict :label "Routing Analyzer" :href "/isomorphism/routing-analyzer")
   (dict :label "Data Test" :href "/isomorphism/data-test")
   (dict :label "Async IO" :href "/isomorphism/async-io")
-  (dict :label "Streaming" :href "/isomorphism/streaming")))
+  (dict :label "Streaming" :href "/isomorphism/streaming")
+  (dict :label "Affinity" :href "/isomorphism/affinity")))
 
 (define plans-nav-items (list
   (dict :label "Status" :href "/plans/status"