The frame diff was comparing byte at a time; compare words, and pay the bridge on every frame

Two findings, both in the P4 shell's own `present`.

## std.mem.eql was the largest read in the firmware, one byte at a time

The shadow-grid diff compares each row against the previous frame: two 13 KB streams,
every frame, and by far the biggest memory access the firmware makes. It measured 3.2
cycles per byte, which is about four times what word-wide loads need - the shape of a
byte-at-a-time loop, and `std.mem.eql` is what it was.

`sameBytes` compares a `u32` at a time and falls back to the byte loop when the spans are
not aligned for it. The alignment test has to be a RUNTIME one because `Cell` is all `u8`
fields and therefore has alignment 1: whether a row begins on a word boundary is a
property of whoever allocated the Surface, not of the type. A row is 40 cells of 26 bytes,
divisible by four, so an aligned base makes every row aligned.

The answer is bit-for-bit the same - this is still exact byte equality - so it keeps the
property the whole diff rests on: byte equality implies visual equality, so the diff can
never claim two different cells are the same.

Measured on the die: the grid walk 223 -> 66 us, 0.95 cycles per byte. 157 us off every
keystroke at every document length, and the single largest win since the clock raise.

## A frame that only hides the cursor still has to fill a USB packet

The padding added for the bridge's 32-byte bulk-IN packet covered the branch that
positions the cursor and not the branch that hides it. A frame that only hid the cursor
was six bytes and waited out the bridge's timer. Hiding an already-hidden cursor is as
idempotent as positioning it twice, so it pads the same way.

The packet size is no longer inferred from an experiment either: 32 is `wMaxPacketSize` of
endpoint 0x82 as the device reports it, and the sweep over pad targets confirms what it
implies - 0 and 16 sit at 4.7-5.1 ms, while 32, 48 and 64 all sit at 3.6-3.8 ms. Crossing
the boundary is worth about 950 us; going past it buys nothing.

## Result

    length   0     20     40     80    160    320    640 chars
    RTT   3602   3624   3638   3790   3868   4026   4192 us

Fixed cost 3652 us against a 4 ms target, from 16.99 ms where this started. A
phase-randomised instrument agrees over 80 trials: median 3687 us, minimum 3571, maximum
3912 - every trial under 4 ms.

The two lengths still above 4 ms are the ones where the line has outgrown the viewport, so
the cursor is off screen and the keystroke changes NOTHING: the frame is 36 bytes of
cursor-hide and padding, zero cells changed, while pardes still rebuilds all 480 cells of
the Surface for 858-985 us. That is the one architectural item left and it is not a micro
-optimisation: nothing in this repository can avoid work pardes has already done.

Verified: screen byte-identical to the vaxis reference on the 18-step workload, with
canonical style decoding rather than escape history. snap 95/95, hxdiff 481/0, hxparity
561/0, unit-test, both A/B arms build, tty, p4 and gui all build.

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