Fix VM correctness: get nil-safe, scope/context/collect! as primitives

- get primitive returns nil for type mismatches (list+string) instead
  of raising — matches JS/Python behavior, fixes find-nav-match errors
- scope-peek, collect!, collected, clear-collected! registered as real
  primitives in sx_primitives table (not just env bindings) so the CEK
  step-sf-context can find them via get-primitive
- step-sf-context checks scope-peek hashtable BEFORE walking CEK
  continuation — bridges aser's scope-push!/pop! with CEK's context
- context, emit!, emitted added to SPECIAL_FORM_NAMES and handled in
  aser-special (scope operations in aser rendering mode)
- sx-context NativeFn for VM-compiled code paths
- VM execution errors no longer mark functions as permanently failed —
  bytecode is correct, errors are from runtime data
- kbd, samp, var added to HTML_TAGS + sx-browser.js rebuilt

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/adapter-sx.sx

@@ -291,7 +291,8 @@
         "defhandler" "defpage" "defquery" "defaction" "defrelation"
         "begin" "do" "quote" "quasiquote"
         "->" "set!" "letrec" "dynamic-wind" "defisland"
-        "deftype" "defeffect" "scope" "provide"))
+        "deftype" "defeffect" "scope" "provide"
+        "context" "emit!" "emitted"))
 
 (define HO_FORM_NAMES
   (list "map" "map-indexed" "filter" "reduce"
@@ -460,6 +461,27 @@
             (scope-pop! prov-name)
             result)
 
+        ;; context — scope lookup (uses hashtable stack, not CEK kont)
+        (= name "context")
+          (let ((ctx-name (trampoline (eval-expr (first args) env)))
+                (default-val (if (>= (len args) 2)
+                               (trampoline (eval-expr (nth args 1) env))
+                               nil)))
+            (let ((val (scope-peek ctx-name)))
+              (if (nil? val) default-val val)))
+
+        ;; emit! — scope accumulator
+        (= name "emit!")
+          (let ((emit-name (trampoline (eval-expr (first args) env)))
+                (emit-val (trampoline (eval-expr (nth args 1) env))))
+            (scope-emit! emit-name emit-val)
+            nil)
+
+        ;; emitted — collect accumulated scope values
+        (= name "emitted")
+          (let ((emit-name (trampoline (eval-expr (first args) env))))
+            (or (scope-peek emit-name) (list)))
+
         ;; Everything else — evaluate normally
         :else
           (trampoline (eval-expr expr env))))))