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>

shared/tests/test_sx_app_pages.py

@@ -165,6 +165,19 @@ ATTRIBUTES = [
     "sx-media",
     "sx-disable",
     "sx-on",       # URL slug for sx-on:*
+    "sx-boost",
+    "sx-preload",
+    "sx-preserve",
+    "sx-indicator",
+    "sx-validate",
+    "sx-ignore",
+    "sx-optimistic",
+    "sx-replace-url",
+    "sx-disabled-elt",
+    "sx-prompt",
+    "sx-params",
+    "sx-sse",
+    "sx-sse-swap",
     "sx-retry",
     "data-sx",
     "data-sx-env",
@@ -441,3 +454,24 @@ class TestReferenceAPIs:
     def test_flaky(self):
         r = _get("/reference/api/flaky")
         assert r.status_code in (200, 503)
+
+    def test_prompt_echo(self):
+        r = httpx.get(
+            f"{SX_BASE}/reference/api/prompt-echo",
+            headers={**HEADERS, "SX-Prompt": "Alice"},
+            timeout=TIMEOUT,
+        )
+        assert r.status_code == 200
+        assert "Alice" in r.text
+
+    def test_sse_time(self):
+        """SSE endpoint returns event-stream content type."""
+        with httpx.stream("GET", f"{SX_BASE}/reference/api/sse-time",
+                          headers=HEADERS, timeout=TIMEOUT) as r:
+            assert r.status_code == 200
+            ct = r.headers.get("content-type", "")
+            assert "text/event-stream" in ct
+            # Read just the first chunk to verify format
+            for chunk in r.iter_text():
+                assert "event:" in chunk or "data:" in chunk
+                break  # only need the first chunk