Treat key combinations as a unit (combos)

The display now only updates on key-down (the combo grows as you press)
and when the last key is released (the empty payload starts the linger
with the last full combo). Intermediate releases no longer shrink the
display, so the order you let go of a shortcut doesn't matter: Ctrl+Shift+N
stays Ctrl+Shift+N whether Ctrl, Shift, or N goes up first, and the 1s
linger shows the full combo instead of a partial release state. Matches
the keybinding-tutorial use case better than keyviz, which shrinks
key-by-key on spaced releases.

README.md

@@ -104,12 +104,17 @@ plugin, not just this one.
 - **Typing** — shows just the character: `g`, `G`, `!`, `5`.
 - **Modifier combos** — shows every modifier plus the key: `Super Shift G`,
   `Ctrl C`, `Super Enter`.
+- **Combos are a unit** — a combination stays intact no matter the order you
+  release it: press `Ctrl Shift N`, let go of `Ctrl` first, then `Shift`,
+  then `N`, and the display keeps showing `Ctrl Shift N` the whole time
+  (keyviz shrinks key-by-key; this plugin treats the chord as one unit).
 - **Non-printing keys** — labeled: `Esc`, `Tab`, `F1`, arrows, `Space`.
 - 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.
 - After the last key is released the combo lingers briefly (1s by default,
-  keyviz-style "Duration") and then vanishes in one frame — no fade.
+  keyviz-style "Duration") and then vanishes in one frame — no fade. The
+  lingering combo is always the last full one, never a partial release state.
 - In `mode: "bindings"` only combos with a modifier are shown; plain typing
   stays off screen.