feat(resize): mid-stream resolution resize — host hardening (H1–H5) + session-binary Match window (C1)

design/midstream-resolution-resize.md Phase 0 + Phase 1.

Host (Phase 0):
- H1/H5: per-backend Reconfigure acceptance gate — reject for gamescope
  (all sub-modes; a resize must never relaunch the title) and under the
  per-client-mode identity policy (a resize would resolve a different
  display slot). Synthetic stays reconfigurable on purpose (the protocol
  test source; the C-ABI roundtrip test rides it). Plus a 500 ms host-side
  min-interval backstop against Reconfigure spam.
- H2: rollback/corrective ack — the data plane reports the mode actually
  live after a failed rebuild (or a refresh the backend capped) through a
  reconfig_result channel; the control task forwards it as a second
  accepted Reconfigured so the client's mode slot self-corrects.
- H3: live stats mode — SendStats reads a packed AtomicU64 (w|h|hz)
  updated on every switch instead of latching the session-start mode.
- H4: registry::retire(gen) — a mode-switch rebuild force-releases the
  superseded Linux display, so linger/forever keep-alive policies don't
  accumulate kept monitors at stale modes. VirtualOutput carries pool_gen
  (fresh AND reused) and the Pipeline tuple threads it to the switch arm.

Client (Phase 1, default off):
- Settings: match_window policy + persisted last window size; exposed as
  the Resolution tri-state (Native / Match window / explicit) in the Skia
  console, GTK and WinUI settings pages.
- pf-presenter: window opens at the persisted size; Hello mode follows the
  window's pixel size; D2 trigger discipline (400 ms debounce to
  resize-end, ≥1 s spacing, even-floor + ≥320×200 clamp, each distinct
  size requested at most once — covers rejects and host rollbacks) as a
  pure, unit-tested decision; HUD line + title refresh on a switch.
- Session binary wires both --connect and --browse paths; the WinUI shell
  is session-always, so this covers Windows too.

Verified: workspace tests + clippy green; synthetic --remode end-to-end;
live session-binary run (window at persisted 1000×600 → Hello 1000×600@60).
On-glass per-backend matrix (Mutter/KWin/gamescope-reject, keep-alive
accumulation) still pending before any default flip.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/linux/src/ui_settings.rs

@@ -264,21 +264,23 @@ pub fn show(
     let page = adw::PreferencesPage::new();
 
     let stream = adw::PreferencesGroup::builder().title("Stream").build();
-    let res_names: Vec<String> = RESOLUTIONS
-        .iter()
-        .map(|&(w, h)| {
-            if w == 0 {
-                "Native display".to_string()
-            } else {
-                format!("{w} × {h}")
-            }
-        })
+    // The D1 tri-state: Native, Match window (a virtual index 1, stored as the
+    // `match_window` flag), then the explicit sizes.
+    let res_names: Vec<String> = std::iter::once("Native display".to_string())
+        .chain(std::iter::once("Match window".to_string()))
+        .chain(
+            RESOLUTIONS
+                .iter()
+                .skip(1)
+                .map(|&(w, h)| format!("{w} × {h}")),
+        )
         .collect();
     let res_row = ChoiceRow::new(
         &dialog,
         inline,
         "Resolution",
-        "The host creates a virtual output at exactly this size",
+        "The host creates a virtual output at exactly this size — Match window follows \
+         the stream window, including mid-stream resizes",
         &res_names.iter().map(String::as_str).collect::<Vec<_>>(),
     );
     let hz_names: Vec<String> = REFRESH
@@ -470,10 +472,15 @@ pub fn show(
     // Seed from the current settings.
     {
         let s = settings.borrow();
-        let res_i = RESOLUTIONS
-            .iter()
-            .position(|&(w, h)| w == s.width && h == s.height)
-            .unwrap_or(0);
+        let res_i = if s.match_window {
+            1
+        } else {
+            RESOLUTIONS
+                .iter()
+                .position(|&(w, h)| w == s.width && h == s.height)
+                .map(|i| if i == 0 { 0 } else { i + 1 })
+                .unwrap_or(0)
+        };
         res_row.set_selected(res_i as u32);
         let hz_i = REFRESH.iter().position(|&r| r == s.refresh_hz).unwrap_or(0);
         hz_row.set_selected(hz_i as u32);
@@ -508,8 +515,14 @@ pub fn show(
     dialog.add(&page);
     dialog.connect_closed(move |_| {
         let mut s = settings.borrow_mut();
-        let (w, h) = RESOLUTIONS[(res_row.selected() as usize).min(RESOLUTIONS.len() - 1)];
-        (s.width, s.height) = (w, h);
+        // Index 1 is the virtual "Match window" option; 0 = Native, 2.. = explicit.
+        let res_i = (res_row.selected() as usize).min(RESOLUTIONS.len());
+        s.match_window = res_i == 1;
+        (s.width, s.height) = if res_i <= 1 {
+            (0, 0)
+        } else {
+            RESOLUTIONS[res_i - 1]
+        };
         s.refresh_hz = REFRESH[(hz_row.selected() as usize).min(REFRESH.len() - 1)];
         s.bitrate_kbps = (bitrate_row.value() * 1000.0) as u32;
         s.gamepad = GAMEPADS[(pad_row.selected() as usize).min(GAMEPADS.len() - 1)].to_string();