Port · Calendar: a real exit clears the preview after a key

The calendar's grid clears its preview on leave, and ignores a leave
that follows a key. gpui records the exit a key makes before the
handler runs, so the guard swallowed it and a real exit afterwards
had no change left to report: the stale day stayed the preview. The
grid now listens independently of input, while its days keep the
default, so a key is no leave and a real exit is. A test leaves the
grid after an arrow, a letter and a month key.

shaped()'s doc goes back to its one line; AGENTS.md keeps the why.

TASKS.md

@@ -476,3 +476,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T44a | 1 | FAIL | twelve roots in nine components still said w_full: a root held in a variable, a match arm or an animation passed the sweep |
 | T44a | 2 | FAIL | roots built in render(mut self), returned by shared helpers or passed through them still said w_full: fields, charts, state views, Steps, previews; five measured at content width in a padded card |
 | T44a | 3 | PASS | the brief counted 43 components in the new tests; they hold 41 |
+| T44b | 1 | FAIL | after a key, leaving the calendar grid kept a stale preview, since the guard swallowed the exit gpui records for the key; the shaped() doc grew to 40 words |