Make the chrome fade a build option, and compile it out for the board

A theme change moves the anchored chrome palette - taglines, boxes, line numbers, scroll
bars - from the old colors to the new ones over ten display frames. On a screen that
repaints in microseconds that is a short legible transition, and it is why the code exists:
a palette that teleports reads as a glitch.

On a 115200 serial line it is not a fade. Each of the ten steps recolors every anchored
cell, so the diff finds the whole chrome dirty and spends a frame's worth of wire on it, ten
times over, with nothing else on screen to look at. Measured on the die, one `NextColor`:

    fade on    12,593 bytes   1,097 ms of saturated wire
    fade off    2,425 bytes     215 ms

A second of the editor talking to itself about a color, on the one transport where a second
is noticeable, for a gradient nobody can watch arrive at 11.5 KB/s.

## Comptime, so the code is not there

`ChromeAnimation` now selects between `animation.Transition` and a new `animation.Immediate`
- the same interface with the animation taken out, a value that is only ever what it was
last set to. That is what makes `ChromeTheme.interpolate` unreachable, and unreachable is
what makes it absent: the flashed image drops 2,336 bytes, and the object 13,180.

A bool tested at runtime would have kept every one of those bytes and still paid the
branch. It also would have needed a second meaning bolted onto `animate_theme_changes`,
whose job is the startup window and nothing else; that field is untouched here.

The option is `-Dtheme-animation`, defaulting to off for `p4` and on everywhere else, and it
is an option rather than a platform test because "is a frame expensive" is a property of the
transport: a P4 driven over something faster than a UART would want the fade back, and
`-Dtheme-animation=true` gives it to them.

## What was checked

`Immediate` is new code with one contract worth pinning, and it is the one a caller could
get wrong: it must arrive at the SAME palette a completed fade arrives at. An endpoint that
differed by a rounding step would make the option a change of colors rather than a change of
how long they take. Tested against a fully advanced `Transition` in `animation.zig`.

Full suite: unit-test, snap 95/95, hxdiff 481/0, hxparity 561/0, image-harness, pdf-harness,
mupdf-check. Builds: tty, p4, gui, and tty/gui with the fade forced off. On the die the
canonical verifier reports the screen IDENTICAL across both arms - the workload contains no
theme change, so this is the check that ordinary rendering was not perturbed - and
`p4-bench --check` stays 4/4.

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