fix(input): pointer lock + the missing per-iteration motion flush

Two capture bugs compounding into 'sluggish and not captured':

1. flush_motion was never called from the run loop — the GTK client's
   frame-clock tick flushed pending motion every frame, and the port
   lost it. Pure mouse movement only reached the host when a click/key/
   scroll happened to flush it; the host cursor simply didn't follow.
   Now one coalesced MouseMove per loop iteration.

2. Capture forwarded ABSOLUTE letterboxed coordinates with no pointer
   confinement (GTK parity, the plan's deferred 'stage-2' item): the
   visible local cursor outran the host cursor by the full e2e and
   escaped the window. Capture is now real pointer lock — SDL relative
   mouse mode (hidden, confined, raw deltas) with relative MouseMove on
   the wire, so the host cursor is the only cursor.

Also: focus-gain re-engages after an auto-release (Alt-Tab undoes
itself; the startup focus race can no longer strand the session
uncaptured) while a chord release stays released until the user opts
back in.

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

crates/punktfunk-host/src/mgmt.rs

@@ -2657,7 +2657,7 @@ mod tests {
         let arr = body.as_array().expect("array");
         // Every backend the host knows, in stable order.
         let ids: Vec<&str> = arr.iter().map(|c| c["id"].as_str().unwrap()).collect();
-        assert_eq!(ids, ["kwin", "gamescope", "mutter", "wlroots"]);
+        assert_eq!(ids, ["kwin", "gamescope", "mutter", "wlroots", "hyprland"]);
         for c in arr {
             assert!(c["available"].is_boolean());
             assert!(c["default"].is_boolean());