Draw a frame only when there is one worth drawing

A 9P round trip on a local Unix socket cost 9.7 ms against the SDL shell and
0.758 ms against the terminal one. The server was not slow and the wake was
not broken: instrumenting the path showed every request waking the loop early
(wakes=210, woke_early=212, timed_out=38 over 250 ticks) and being answered on
that same pass. The cost was that `pump` answers 9P at one point in a loop
that then renders and presents unconditionally, so a client's next request
landed while the main thread was blocked on the display, and each round trip
therefore cost a whole frame. The frame rate was governing something that has
nothing to do with drawing.

`Pardes.needs_frame` starts true, is set by every event except a tick with
nothing animating and a filesystem request that only reads, and is cleared
once a frame is presented. `pump` returns before render and present when it is
false and nothing is animating. An idle editor answering reads now draws
nothing at all.

  9P read_fid, one RPC:   gui 9.7 ms -> 0.056 ms (173x)
                          tty 0.758 ms -> 0.062 ms (12x)

Verified the shells still paint rather than going quiet: the rendered screen
carries the opened file, a write through 9P redraws within the frame, and
`fs-discovery-test` passes over the real wire. Suite unchanged at 778/783 with
the two pre-existing crashes.

Also from the adversarial review of the previous commits:

`pardes --tty FILE` silently discarded the file, and `--tty MISSING` silently
discarded the error pane. main.zig named the boot layout before the positional
was resolved, and naming one short-circuits `Boot.of`. The choice now happens
after the argument is known, and only when there is no file and no missing
word. macos.zig names the same layout, so the app no longer boots a different
one from the terminal and SDL shells.

`pre_close_last_pane_tail` was transcribed from the NEW default rather than the
old one, so the upgrade path it was added for did not exist: a workspace dumped
before the tagline reorder came back with the old default welded on as a custom
tail. It is now the string it claims to be.

A pane two rows tall lost its message and, worse, its prompt and the cursor
with it. The notice cap keeps the LAST notices now, because the prompt is last
and a prompt you cannot see is one you type into blind.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

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