Adopt Step 7 language features across SX codebase

112 conversions across 19 .sx files using match, let-match, and pipe operators:

match (17): type/value dispatch replacing cond/if chains
  - lib/vm.sx: HO form dispatch (for-each/map/filter/reduce/some/every?)
  - lib/tree-tools.sx: node-display, node-matches?, rename, count, replace, free-symbols
  - lib/types.sx: narrow-type, substitute-in-type, infer-type, resolve-type
  - web/engine.sx: default-trigger, resolve-target, classify-trigger
  - web/deps.sx: scan-refs-walk, scan-io-refs-walk

let-match (89): dict destructuring replacing (get d "key") patterns
  - shared/page-functions.sx (20), blog/admin.sx (17), pub-api.sx (13)
  - events/ layouts/page/tickets/entries/forms (27 total)
  - specs-explorer.sx (7), federation/social.sx (3), lib/ small files (3)

-> pipes (6): replacing triple-chained gets in lib/vm.sx
  - frame-closure → closure-code → code-bytecode chains

Also: lib/vm.sx accessor upgrades (get vm "sp" → vm-sp vm throughout)

2650/2650 tests pass, zero regressions.

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

events/sx/forms.sx

@@ -323,28 +323,43 @@
 ;; ---------------------------------------------------------------------------
 
 ;; Day checkboxes from data — replaces Python loop
-(defcomp ~forms/day-checkboxes-from-data (&key days-data all-checked)
+(defcomp
+  ~forms/day-checkboxes-from-data
+  (&key days-data all-checked)
   (<>
     (~forms/day-all-checkbox :checked (when all-checked "checked"))
-    (map (lambda (d)
-      (~forms/day-checkbox
-        :name (get d "name")
-        :label (get d "label")
-        :checked (when (get d "checked") "checked")))
+    (map
+      (lambda
+        (d)
+        (let-match
+          {:checked checked :label label :name name}
+          d
+          (~forms/day-checkbox
+            :name name
+            :label label
+            :checked (when checked "checked"))))
       (or days-data (list)))))
 
 ;; Slot options from data — replaces _slot_options_html Python loop
-(defcomp ~forms/slot-options-from-data (&key slots)
-  (<> (map (lambda (s)
-    (~forms/slot-option
-      :value (get s "value")
-      :data-start (get s "data-start")
-      :data-end (get s "data-end")
-      :data-flexible (get s "data-flexible")
-      :data-cost (get s "data-cost")
-      :selected (get s "selected")
-      :label (get s "label")))
-    (or slots (list)))))
+(defcomp
+  ~forms/slot-options-from-data
+  (&key slots)
+  (<>
+    (map
+      (lambda
+        (s)
+        (let-match
+          {:data-end data-end :data-flexible data-flexible :selected selected :value value :data-cost data-cost :label label :data-start data-start}
+          s
+          (~forms/slot-option
+            :value value
+            :data-start data-start
+            :data-end data-end
+            :data-flexible data-flexible
+            :data-cost data-cost
+            :selected selected
+            :label label)))
+      (or slots (list)))))
 
 ;; Slot picker from data — wraps picker + options
 (defcomp ~forms/slot-picker-from-data (&key id slots)