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/ref/test-eval.sx

@@ -396,7 +396,32 @@
   (deftest "component with default via or"
     (defcomp ~label (&key text)
       (span (or text "default")))
-    (assert-true (not (nil? ~label)))))
+    (assert-true (not (nil? ~label))))
+
+  (deftest "defcomp default affinity is auto"
+    (defcomp ~aff-default (&key x)
+      (div x))
+    (assert-equal "auto" (component-affinity ~aff-default)))
+
+  (deftest "defcomp affinity client"
+    (defcomp ~aff-client (&key x)
+      :affinity :client
+      (div x))
+    (assert-equal "client" (component-affinity ~aff-client)))
+
+  (deftest "defcomp affinity server"
+    (defcomp ~aff-server (&key x)
+      :affinity :server
+      (div x))
+    (assert-equal "server" (component-affinity ~aff-server)))
+
+  (deftest "defcomp affinity preserves body"
+    (defcomp ~aff-body (&key val)
+      :affinity :client
+      (span val))
+    ;; Component should still render correctly
+    (assert-equal "client" (component-affinity ~aff-body))
+    (assert-true (not (nil? ~aff-body)))))
 
 
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