Add attribute detail pages with live demos for SX reference

Per-attribute documentation pages at /reference/attributes/<slug> with:
- Live interactive demos (demo components in reference.sx)
- S-expression source code display
- Server handler code shown as s-expressions (defhandlers in handlers/reference.sx)
- Wire response display via OOB swaps on demo interaction
- Linked attribute names in the reference table

Covers all 20 implemented attributes (sx-get/post/put/delete/patch,
sx-trigger/target/swap/swap-oob/select/confirm/push-url/sync/encoding/
headers/include/vals/media/disable/on:*, sx-retry, data-sx, data-sx-env).

Also adds sx-on:* to BEHAVIOR_ATTRS, updates REFERENCE_NAV to link
/reference/attributes, and makes /reference/ an index page.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sxc/handlers/reference.sx

@@ -0,0 +1,149 @@
+;; SX reference — defhandler definitions for attribute detail demos
+;;
+;; These serve the live demos on the Reference > Attributes detail pages.
+
+;; ---------------------------------------------------------------------------
+;; Shared: return server time
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-time (&key)
+  (let ((now (format-time (now) "%H:%M:%S")))
+    (span :class "text-stone-800 text-sm"
+      "Server time: " (strong now))))
+
+;; ---------------------------------------------------------------------------
+;; sx-post: greet
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-greet (&key)
+  (let ((name (or (form-data "name") "stranger")))
+    (span :class "text-stone-800 text-sm"
+      "Hello, " (strong name) "!")))
+
+;; ---------------------------------------------------------------------------
+;; sx-put: update status
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-status (&key)
+  (let ((status (or (form-data "status") "unknown")))
+    (span :class "text-stone-700 text-sm"
+      "Status: " (strong status) " — updated via PUT")))
+
+;; ---------------------------------------------------------------------------
+;; sx-patch: update theme
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-theme (&key)
+  (let ((theme (or (form-data "theme") "unknown")))
+    (str theme)))
+
+;; ---------------------------------------------------------------------------
+;; sx-delete: remove item
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-delete (&key item-id)
+  "")
+
+;; ---------------------------------------------------------------------------
+;; sx-trigger: search
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-trigger-search (&key)
+  (let ((q (or (request-arg "q") "")))
+    (if (empty? q)
+      (span :class "text-stone-400 text-sm" "Start typing to trigger a search.")
+      (span :class "text-stone-800 text-sm"
+        "Results for: " (strong q)))))
+
+;; ---------------------------------------------------------------------------
+;; sx-swap: new item
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-swap-item (&key)
+  (let ((now (format-time (now) "%H:%M:%S")))
+    (div :class "text-sm text-violet-700"
+      "New item (" now ")")))
+
+;; ---------------------------------------------------------------------------
+;; sx-swap-oob: update two targets
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-oob (&key)
+  (let ((now (format-time (now) "%H:%M:%S")))
+    (<>
+      (span :class "text-emerald-700 text-sm"
+        "Main updated at " now)
+      (div :id "ref-oob-side" :sx-swap-oob "innerHTML"
+        (span :class "text-violet-700 text-sm"
+          "OOB updated at " now)))))
+
+;; ---------------------------------------------------------------------------
+;; sx-select: page with multiple sections
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-select-page (&key)
+  (let ((now (format-time (now) "%H:%M:%S")))
+    (<>
+      (div :id "the-header"
+        (h3 "Page header — not selected"))
+      (div :id "the-content"
+        (span :class "text-emerald-700 text-sm"
+          "This fragment was selected from a larger response. Time: " now))
+      (div :id "the-footer"
+        (p "Page footer — not selected")))))
+
+;; ---------------------------------------------------------------------------
+;; sx-sync: slow echo
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-slow-echo (&key)
+  (sleep 800)
+  (let ((q (or (request-arg "q") "")))
+    (span :class "text-stone-800 text-sm"
+      "Echo: " (strong q))))
+
+;; ---------------------------------------------------------------------------
+;; sx-encoding: file upload name
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-upload-name (&key)
+  (let ((name (or (file-name "file") "(no file)")))
+    (span :class "text-stone-800 text-sm"
+      "Received: " (strong name))))
+
+;; ---------------------------------------------------------------------------
+;; sx-headers: echo custom headers
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-echo-headers (&key)
+  (let ((headers (request-headers :prefix "X-")))
+    (if (empty? headers)
+      (span :class "text-stone-400 text-sm" "No custom headers received.")
+      (ul :class "text-sm text-stone-700 space-y-1"
+        (map (fn (h)
+          (li (strong (first h)) ": " (last h)))
+          headers)))))
+
+;; ---------------------------------------------------------------------------
+;; sx-include / sx-vals: echo all values
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-echo-vals (&key)
+  (let ((vals (request-args)))
+    (if (empty? vals)
+      (span :class "text-stone-400 text-sm" "No values received.")
+      (ul :class "text-sm text-stone-700 space-y-1"
+        (map (fn (v)
+          (li (strong (first v)) ": " (last v)))
+          vals)))))
+
+;; ---------------------------------------------------------------------------
+;; sx-retry: flaky endpoint
+;; ---------------------------------------------------------------------------
+
+(defhandler ref-flaky (&key)
+  (let ((n (inc-counter "ref-flaky")))
+    (if (!= (mod n 3) 0)
+      (error 503)
+      (span :class "text-emerald-700 text-sm"
+        "Success on attempt " n "!"))))