A keystroke is now 8.37 ms, from 16.99; the remaining 6.4 ms is located

Progression on the die, five document lengths x seven trials at each step:

    step                fixed    per char   at 160 chars
    ReleaseSmall      16.99 ms    54.3 us      25.56 ms
    ReleaseFast       14.85 ms    34.7 us      20.30 ms  0.79x
    + ASCII grapheme  14.56 ms    12.0 us      16.46 ms  0.64x
    + ASCII print     14.27 ms     6.9 us      15.36 ms  0.60x
    + shadow grid      8.87 ms     7.3 us      10.02 ms  0.39x
    + byte compare     8.37 ms     7.1 us       9.48 ms  0.37x

Round trip is time to the FIRST response byte: ~1.95 ms of host and USB latency plus
compute. Compute is 6.42 ms and the 4 ms goal needs it under 2.05 ms, so 3.1x remains.

Every microsecond of it is now measured rather than guessed, by stage, on the die:

    our walk of the grid        979 us   this repo's own present()
    vaxis diff + emit          2455 us   vaxis walks all 480 cells, per-cell strings
    pardes Surface rebuild     2000 us   pardes rebuilds every cell every frame
    input parse + edit          250 us

The honest reading is that the two large items are not each other's alternative.
vaxis's diff is redundant work - present() already computes exactly which cells moved,
so emitting ANSI directly from the shadow grid would remove most of that 2.46 ms. But
even a perfect renderer leaves pardes rebuilding a whole Surface per keystroke, and 4 ms
at 40x12 needs that too.

Raising the baud does not move this number. At 115200 an 81-byte reply is 7.0 ms of
wire, but almost none of it lands before the first byte; 921600 takes `settle` from
23 ms to ~16 ms and leaves the round trip where it is. Worth doing, not for this.

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