Support isolated live window ScreenCast streams

Render selected toplevels through a private portal-only capture protocol, preserve logical snapshot geometry, and renegotiate PipeWire video on window resizing. Bound outstanding snapshots and retry temporary contention; stop on source closure and session locking.

Validation: portal unit tests (31), capture budget unit, private-bus portal contracts, and real nested/PipeWire isolation, workspace, resize, close and paused-lock tests pass. Independent review cleared. Monitor restore integration timed out on both this build and the unchanged previous compositor under the current nested host.

scripts/tests/fixtures/session-lock.c

@@ -0,0 +1,44 @@
+#include <stdio.h>
+#include <string.h>
+#include <wayland-client.h>
+#include "ext-session-lock-client-protocol.h"
+
+static struct ext_session_lock_manager_v1 *manager;
+
+static void global(void *data, struct wl_registry *registry, uint32_t name,
+                   const char *interface, uint32_t version) {
+    (void)data; (void)version;
+    if (!strcmp(interface, "ext_session_lock_manager_v1"))
+        manager = wl_registry_bind(registry, name, &ext_session_lock_manager_v1_interface, 1);
+}
+
+static void removed(void *data, struct wl_registry *registry, uint32_t name) {
+    (void)data; (void)registry; (void)name;
+}
+
+static void locked(void *data, struct ext_session_lock_v1 *lock) {
+    (void)data; (void)lock;
+    puts("locked");
+    fflush(stdout);
+}
+
+static void finished(void *data, struct ext_session_lock_v1 *lock) {
+    (void)data; (void)lock;
+}
+
+int main(void) {
+    struct wl_display *display = wl_display_connect(NULL);
+    if (!display) return 1;
+    const struct wl_registry_listener registry_listener = {global, removed};
+    struct wl_registry *registry = wl_display_get_registry(display);
+    wl_registry_add_listener(registry, &registry_listener, NULL);
+    if (wl_display_roundtrip(display) < 0 || !manager) return 2;
+    struct ext_session_lock_v1 *lock = ext_session_lock_manager_v1_lock(manager);
+    const struct ext_session_lock_v1_listener listener = {locked, finished};
+    ext_session_lock_v1_add_listener(lock, &listener, NULL);
+    if (wl_display_roundtrip(display) < 0) return 3;
+    puts("requested");
+    fflush(stdout);
+    while (wl_display_dispatch(display) != -1) {}
+    return 0;
+}