Raise the board's grid to 56x14, and make it a build option

The 40x12 ceiling was never about the screen. It was about memory, and the comment above
`max_cols` said so: "every cell is paid for four times over: vaxis keeps a Screen and an
InternalScreen, pardes keeps its own Surface and previous_cells". Two of those four are now
dead weight - with `direct_emit` the emitter diffs the Surface against its own shadow and
writes the escapes itself, so vaxis's two grids are allocated, never read, and were the
largest single claim on a 384 KiB heap. `init` sizes them to ONE CELL. vaxis still does the
work only it can do: the alternate screen, the capability queries, and parsing everything
that comes back.

That removes the memory ceiling entirely - the heap now reports 336 KB free at every
geometry tried, including ones that used to fail - and leaves latency as the only limit,
which is the honest one: every frame walks the whole grid.

## Measured on the die, 0.87 us per cell

    geometry   cells   round trip
      40x12      480     3,628 us   the old default
      56x14      784     3,930 us   the new one
      56x16      896     3,965 us
      60x18    1,080     4,114 us
      64x20    1,280     4,281 us
      80x24    1,920     4,809 us
     100x30    3,000     5,743 us
     120x36    4,320     6,923 us
     140x42    5,880     8,310 us   the largest that runs
     160x48    7,680     links, then traps
     200x60   12,000     does not link

56x14 is 63% more area and 40% more width than 40x12 and still holds the 4 ms this port was
built to. 56x16 was tried first: 3,965 us on the bench instrument but 4,029 on the
phase-randomised one, which is over, and the two instruments differ by about 50 us
systematically - so the wider grid went and two rows stayed behind. Width is worth more than
height for reading code.

The two failures at the top are worth naming precisely because they are different failures.
200x60 does not link: `.bss will not fit in region l2mem, overflowed by 76036 bytes`, that
`.bss` being the shell's shadow copy of the grid, sized at comptime. 160x48 links and then
TRAPS at boot - the same region pressure arriving at runtime as a collision rather than as a
diagnostic. Neither is a heap problem any more, which is the interesting part: the heap has
336 KB spare while `.bss` runs out.

`-Dp4-cols` / `-Dp4-rows` because none of the above is a constant. 80x24 is one flag away for
anyone who would rather have the classic terminal than the millisecond.

Verified at the new geometry rather than assumed: the A/B against the reference path - vaxis
rendering, full repaint, `shadow_grid` and `direct_emit` both off - is identical in every
cell, characters and resolved style. That matters more here than usual because the emitter's
column arithmetic has a special case at the last column, and 40 was the only width it had
ever been asked about. snap 95/95, hxdiff 481/0, hxparity 561/0, unit-test, both A/B arms,
tty/p4/gui, and the board's own `p4-bench --check`.

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