Linger on screen after release, keyviz-style

Previously the chips cleared the instant the keys were released, so the
120ms hide delay was invisible. Keep the last combo rendered for the
whole linger window and only clear it when the timer fires, so a quick
tap stays readable (keyviz keeps keys for lingerDurationMs, 5s by
default). Default 1000ms, tunable via lingerMs.

README.md

@@ -71,7 +71,8 @@ The panel starts with the shell, so it works after the next login too.
 - Modifiers that produce the shifted character (`Shift` with a letter) are
   folded into the character itself: `Shift + 1` shows `!`.
 - Key repeat is ignored; a held key shows once.
-- The combo disappears immediately after the last key is released.
+- After the last key is released the combo lingers briefly (1s by default,
+  keyviz-style "Duration") and then vanishes in one frame — no fade.
 
 ## Customize
 
@@ -81,7 +82,7 @@ automatically.
 
 | Want to change...                 | Edit                                    |
 |-----------------------------------|-----------------------------------------|
-| How long a released combo lingers | `hideDelayMs` in `KeyVisualizer.qml` (default `120`) |
+| How long a released combo lingers | `lingerMs` in `KeyVisualizer.qml` (default `1000`; keyviz defaults to `5000`) |
 | Chip font / padding               | `chipFont`, `chipPadX/Y` in `KeyVisualizer.qml` |
 | Key labels or layout mapping      | `KEYS` / `CHARS` / `SHIFTED` tables in `key-visualizer.lua` |