Add SSE, response headers, view transitions, and 5 new sx attributes

Implement missing SxEngine features:
- SSE (sx-sse, sx-sse-swap) with EventSource management and auto-cleanup
- Response headers: SX-Trigger, SX-Retarget, SX-Reswap, SX-Redirect,
  SX-Refresh, SX-Location, SX-Replace-Url, SX-Trigger-After-Swap/Settle
- View Transitions API: transition:true swap modifier + global config
- every:<time> trigger for polling (setInterval)
- sx-replace-url (replaceState instead of pushState)
- sx-disabled-elt (disable elements during request)
- sx-prompt (window.prompt, value sent as SX-Prompt header)
- sx-params (filter form parameters: *, none, not x,y, x,y)

Adds docs (ATTR_DETAILS, BEHAVIOR_ATTRS, headers, events), demo
components in reference.sx, API endpoints (prompt-echo, sse-time),
and 27 new unit tests for engine logic.

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

sx/sxc/reference.sx

@@ -551,3 +551,95 @@
         :class "text-red-500 text-sm hover:text-red-700" "Remove"))
     (p :class "text-xs text-stone-400"
       "Items fade out immediately on click (optimistic), then are removed when the server responds.")))
+
+;; ---------------------------------------------------------------------------
+;; sx-replace-url
+;; ---------------------------------------------------------------------------
+
+(defcomp ~ref-replace-url-demo ()
+  (div :class "space-y-3"
+    (button
+      :sx-get "/reference/api/time"
+      :sx-target "#ref-replurl-result"
+      :sx-swap "innerHTML"
+      :sx-replace-url "true"
+      :class "px-4 py-2 bg-violet-600 text-white rounded hover:bg-violet-700 transition-colors text-sm"
+      "Load (replaces URL)")
+    (div :id "ref-replurl-result"
+      :class "p-3 rounded bg-stone-50 text-stone-400 text-sm"
+      "Click to load — URL changes but no new history entry.")))
+
+;; ---------------------------------------------------------------------------
+;; sx-disabled-elt
+;; ---------------------------------------------------------------------------
+
+(defcomp ~ref-disabled-elt-demo ()
+  (div :class "space-y-3"
+    (div :class "flex gap-3 items-center"
+      (button :id "ref-diselt-btn"
+        :sx-get "/reference/api/slow-echo"
+        :sx-target "#ref-diselt-result"
+        :sx-swap "innerHTML"
+        :sx-disabled-elt "#ref-diselt-btn"
+        :sx-vals "{\"q\": \"hello\"}"
+        :class "px-4 py-2 bg-violet-600 text-white rounded hover:bg-violet-700 transition-colors text-sm disabled:opacity-50"
+        "Click (disables during request)")
+      (span :class "text-xs text-stone-400" "Button is disabled while request is in-flight."))
+    (div :id "ref-diselt-result"
+      :class "p-3 rounded bg-stone-50 text-stone-400 text-sm"
+      "Click the button to see it disable during the request.")))
+
+;; ---------------------------------------------------------------------------
+;; sx-prompt
+;; ---------------------------------------------------------------------------
+
+(defcomp ~ref-prompt-demo ()
+  (div :class "space-y-3"
+    (button
+      :sx-get "/reference/api/prompt-echo"
+      :sx-target "#ref-prompt-result"
+      :sx-swap "innerHTML"
+      :sx-prompt "Enter your name:"
+      :class "px-4 py-2 bg-violet-600 text-white rounded hover:bg-violet-700 transition-colors text-sm"
+      "Prompt & send")
+    (div :id "ref-prompt-result"
+      :class "p-3 rounded bg-stone-50 text-stone-400 text-sm"
+      "Click to enter a name via prompt — it is sent as the SX-Prompt header.")))
+
+;; ---------------------------------------------------------------------------
+;; sx-params
+;; ---------------------------------------------------------------------------
+
+(defcomp ~ref-params-demo ()
+  (div :class "space-y-3"
+    (form
+      :sx-post "/reference/api/echo-vals"
+      :sx-target "#ref-params-result"
+      :sx-swap "innerHTML"
+      :sx-params "name"
+      :class "flex gap-2"
+      (input :type "text" :name "name" :placeholder "Name (sent)"
+        :class "flex-1 px-3 py-2 border border-stone-300 rounded text-sm focus:outline-none focus:ring-2 focus:ring-violet-500")
+      (input :type "text" :name "secret" :placeholder "Secret (filtered)"
+        :class "flex-1 px-3 py-2 border border-stone-300 rounded text-sm focus:outline-none focus:ring-2 focus:ring-violet-500")
+      (button :type "submit"
+        :class "px-4 py-2 bg-violet-600 text-white rounded hover:bg-violet-700 transition-colors text-sm"
+        "Submit"))
+    (div :id "ref-params-result"
+      :class "p-3 rounded bg-stone-50 text-stone-400 text-sm"
+      "Only 'name' will be sent — 'secret' is filtered by sx-params.")))
+
+;; ---------------------------------------------------------------------------
+;; sx-sse
+;; ---------------------------------------------------------------------------
+
+(defcomp ~ref-sse-demo ()
+  (div :class "space-y-3"
+    (div :sx-sse "/reference/api/sse-time"
+      :sx-sse-swap "time"
+      :sx-swap "innerHTML"
+      (div :id "ref-sse-result"
+        :class "p-3 rounded bg-stone-50 text-stone-600 text-sm font-mono"
+        "Connecting to SSE stream..."))
+    (p :class "text-xs text-stone-400"
+      "Server pushes time updates every 2 seconds via Server-Sent Events.")))