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/eval.sx

@@ -491,17 +491,37 @@
 
 (define sf-defcomp
   (fn (args env)
+    ;; (defcomp ~name (params) [:affinity :client|:server] body)
+    ;; Body is always the last element. Optional keyword annotations
+    ;; may appear between the params list and the body.
     (let ((name-sym   (first args))
           (params-raw (nth args 1))
-          (body       (nth args 2))
+          (body       (last args))
           (comp-name  (strip-prefix (symbol-name name-sym) "~"))
           (parsed     (parse-comp-params params-raw))
           (params     (first parsed))
-          (has-children (nth parsed 1)))
-      (let ((comp (make-component comp-name params has-children body env)))
+          (has-children (nth parsed 1))
+          (affinity   (defcomp-kwarg args "affinity" "auto")))
+      (let ((comp (make-component comp-name params has-children body env affinity)))
         (env-set! env (symbol-name name-sym) comp)
         comp))))
 
+(define defcomp-kwarg
+  (fn (args key default)
+    ;; Search for :key value between params (index 2) and body (last).
+    (let ((end (- (len args) 1))
+          (result default))
+      (for-each
+        (fn (i)
+          (when (and (= (type-of (nth args i)) "keyword")
+                     (= (keyword-name (nth args i)) key)
+                     (< (+ i 1) end))
+            (let ((val (nth args (+ i 1))))
+              (set! result (if (= (type-of val) "keyword")
+                             (keyword-name val) val)))))
+        (range 2 end 1))
+      result)))
+
 (define parse-comp-params
   (fn (params-expr)
     ;; Parse (&key param1 param2 &children) → (params has-children)
@@ -879,7 +899,7 @@
 ;;
 ;; Constructors:
 ;;   (make-lambda params body env)           → Lambda
-;;   (make-component name params has-children body env) → Component
+;;   (make-component name params has-children body env affinity) → Component
 ;;   (make-macro params rest-param body env name) → Macro
 ;;   (make-thunk expr env)                   → Thunk
 ;;
@@ -893,6 +913,7 @@
 ;;   (component-body c)       → expr
 ;;   (component-closure c)    → env
 ;;   (component-has-children? c) → boolean
+;;   (component-affinity c)   → "auto" | "client" | "server"
 ;;   (macro-params m)         → list of strings
 ;;   (macro-rest-param m)     → string or nil
 ;;   (macro-body m)           → expr