Make a keystroke 2.6x cheaper by not asking Unicode about ASCII

A keystroke on the ESP32-P4 cost 17.0 ms and the goal is 4. Profiling the core in
that board's exact configuration - 40x12, tree-sitter disabled, via `zig build perf
-Dtree-sitter=disabled -- --cols 40 --rows 12 --only small` - named the cost, and it
was Unicode machinery answering questions about the letter `y`.

Four changes, each a fast path guarded so that non-ASCII text takes exactly the road
it took before.

`modal.graphemeStart` was 21.5% of a keystroke, the single largest item. It iterates
graphemes FROM THE START of the text with the full UAX #29 break state machine until
it passes the offset, and the render path calls it once per visible row with a column
offset - so the cost followed the cursor's distance along its line. That is the shape
measured on the die, where inserting at column 320 of a fixed 320-character line cost
7.8 ms more than inserting at column 0 of the same line. In UAX #29 every ASCII
scalar is its own cluster with ONE exception, GB3 (CR joined to LF); every other rule
that could extend a cluster - Extend, ZWJ, SpacingMark, Prepend, Regional_Indicator -
is spelled with non-ASCII scalars. So an ASCII byte whose predecessor is also ASCII,
and not that CR-LF pair, IS a boundary. O(1), and sound rather than approximate.

`Surface.print` then became the largest at 26.2%: per character it took a UTF-8
length, a decode, a FRESHLY CONSTRUCTED grapheme iterator, a slice validation and a
width lookup, to conclude that `y` is one cell. Printable ASCII followed by ASCII
takes none of that now. Same guard, same reason.

`file_pane.graphemeDisplayWidth` was 6.9%, essentially all of it asking `gwidth`
about ASCII. Bounded to 0x20..0x7e on purpose: DEL and the C0 controls are not one
printable cell and `gwidth` stays the authority on them.

`modal.lineSlice` searched for "\n" with the generic substring search where a memchr
does; it is called once per visible row per frame.

Measured at the P4's geometry and configuration, on the host: render 55 -> 12 us,
key-down 483 -> 24 us, key-right 327 -> 13 us, edit-char 205 -> 46 us. On the die,
the per-character cost of a keystroke fell from 54.3 to 6.9 us - 7.9x - and a
keystroke at a 160-character line from 25.56 ms to 15.36 ms.

## The shadow grid, and why it is static

`src/p4.zig`'s `present` copied all 480 cells into vaxis every frame, which measured
6.75 ms on the die - 57% of a keystroke - and was paid whether or not anything
changed: a second render with nothing new cost the same as the first. vaxis diffs its
own grid, but only after being told every cell, and being told is the expensive part.
So `present` now keeps the previous Surface and tells vaxis only what moved.
`Cell.visuallyEqual` is the right comparison and already existed. Copy: 6.75 -> 1.45 ms.

The grid lives in `.bss`, sized by `max_cols` x `max_rows` at comptime, and that is
not a micro-optimisation. The first version allocated it from the editor's heap; on a
board whose 384 KiB is nearly spoken for, that is exactly the kind of change that
works and then breaks something else three steps away.

`shadow_grid` is a comptime A/B switch, kept deliberately. With it false, `present`
behaves as it did before - clear and write every cell - which is the reference any
measurement should be compared against, and the way to tell a rendering bug from a
rendering difference. It earned its keep immediately: the two paths were run against
the same 19-step workload on the die - inserts, deletes, motions that move the
modified-marker, a line outgrowing the viewport, backspaces that shrink it - and the
reconstructed screens are byte-identical.

## Verification

`snap` 95/95 scripts, `hxdiff` 481 cases 0 mismatches, `hxparity` 561 cases 0
mismatches, `unit-test`, `image-harness`, `pdf-harness`, `mupdf-check`, and tty / p4 /
gui all build. The rendering changes are exactly the sort that pass a latency
benchmark while corrupting a screen, so the snapshot parity suite is the one that
matters here and it is unchanged.

`test/perf.zig` gains `--cols`/`--rows`/`--only`. The screen's shape is one of the
things that table exists to hold constant, and 40x12 is not a scaled guess at the
board - it is the board. `--only` exists because under `perf record` one 63 ms cell on
the largest fixture swamps every sample from the case being asked about.

## Found, not fixed

`vx.resize` fails on this board: a runtime geometry change hits its allocation
failure path, restores the previous size and returns, so 80 bytes go out where 1,392
should. Verified independent of everything above - it reproduces with `shadow_grid`
false. The board therefore has one geometry for the life of a session, which is why
the staleness test above compares two firmwares rather than resizing one.

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