Compare shadow-grid cells as bytes, not through std.meta.eql

`Cell.visuallyEqual` is the semantically exact answer and too slow to ask 480 times a
frame: `std.meta.eql` on a `CellStyle` recurses through a colour union and eight
booleans, and the walk measured 1.45 ms on the die - about 270 cycles to compare a
28-byte struct.

`sameCell` in src/p4.zig does it as bytes. That is safe in the direction that
matters: byte equality IMPLIES visual equality, so it can never claim two different
cells are the same. It can miss an equality - scratch bytes past `len`, or padding -
and the only cost of that is one redundant `writeCell` which vaxis then diffs away.
Defaults are still compared by meaning, because an unpainted cell's text and style are
whatever the previous frame left in them.

Measured: the grid walk 1.45 -> 0.98 ms, a keystroke 8.87 -> 8.37 ms fixed.

Verified the way a rendering change has to be. The A/B harness now hashes the SGR
state of every cell as well as its character, because the first version compared text
only and would have passed a colour regression in silence. Reference path
(`shadow_grid = false`, clear and write everything) and incremental path were each run
against the same 19-step workload on the die and the reconstructed screens are
identical in both text and per-row style hash.

snap 95/95, hxdiff 481 cases 0 mismatches, hxparity 561 cases 0 mismatches, unit-test,
and tty / p4 / gui all build.

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