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>

spec/evaluator.sx

@@ -1476,19 +1476,29 @@
               (make-provide-frame name val (rest body) env)
               kont)))))))
 
-;; context: walk kont for nearest ProvideFrame with matching name
+;; context: check hashtable scope stacks first (set by aser's scope-push!),
+;; then walk kont for nearest ProvideFrame with matching name.
+;; The hashtable check is needed because aser renders scopes via scope-push!/pop!
+;; but inner eval-expr calls (e.g. inside (str ...)) use the CEK continuation.
 (define step-sf-context
   (fn (args env kont)
     (let ((name (trampoline (eval-expr (first args) env)))
           (default-val (if (>= (len args) 2)
                          (trampoline (eval-expr (nth args 1) env))
-                         nil))
-          (frame (kont-find-provide kont name)))
-      (if frame
-        (make-cek-value (get frame "value") env kont)
-        (if (>= (len args) 2)
-          (make-cek-value default-val env kont)
-          (error (str "No provider for: " name)))))))
+                         nil)))
+      ;; Check hashtable scope stacks first (aser rendering path)
+      (let ((stack-val (if (primitive? "scope-peek")
+                         ((get-primitive "scope-peek") name)
+                         nil)))
+        (if (not (nil? stack-val))
+          (make-cek-value stack-val env kont)
+          ;; Fall back to continuation-based lookup
+          (let ((frame (kont-find-provide kont name)))
+            (if frame
+              (make-cek-value (get frame "value") env kont)
+              (if (>= (len args) 2)
+                (make-cek-value default-val env kont)
+                (error (str "No provider for: " name))))))))))
 
 ;; emit!: walk kont for nearest ScopeAccFrame, append value
 (define step-sf-emit