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.

1 files changed, 22 insertions, 1 deletions

diff --git a/src/pardes.zig b/src/pardes.zig
index 691ee345..9f20e0a3 100644
--- a/src/pardes.zig
+++ b/src/pardes.zig
@@ -68,6 +68,17 @@ pub const frameMark = tracy.frameMark;
 pub const Platform = enum { tty, gui, web, macos, p4 };
 pub const platform: Platform = @field(Platform, @tagName(@import("pardes_config").platform));
 
+/// Whether a theme change FADES the anchored chrome palette or replaces it. Ten display frames
+/// either way (`animation.transition_steps`), and on every screen but one that is a short legible
+/// transition rather than a glitch.
+///
+/// The exception is a screen reached through a UART. 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, for a fade nobody can see arrive gradually anyway. Off by default for `p4` and settable
+/// either way from the build, because the thing that makes it wrong is the transport rather than the
+/// target - see `build.zig`.
+pub const theme_animation = @import("pardes_config").theme_animation;
+
 /// A build with no host but its display: the embedded source filesystem, the
 /// in-process clipboard, silent ptys. Comptime, and its own option module
 /// rather than a `pardes_config` field, because it is the one setting that
@@ -2436,7 +2447,13 @@ pub const ChromeTheme = struct {
     }
 };
 
-const ChromeAnimation = animation.Transition(ChromeTheme);
+/// The fade, or its absence, decided at comptime so that `-Dtheme-animation=false` leaves
+/// `ChromeTheme.interpolate` unreachable and therefore out of the binary entirely. Every call site
+/// below is written against the shared interface and needs no condition of its own.
+const ChromeAnimation = if (theme_animation)
+    animation.Transition(ChromeTheme)
+else
+    animation.Immediate(ChromeTheme);
 const initial_chrome = ChromeTheme.fromTheme(&themes[0]);
 
 // ---- the boundary types ----
@@ -5548,6 +5565,10 @@ pub const Pardes = struct {
     chrome_animation: ChromeAnimation = ChromeAnimation.init(initial_chrome),
     /// False only while startup configuration or a dump restore is selecting
     /// its first theme. No frame is rendered in that interval.
+    ///
+    /// Nothing to do with `-Dtheme-animation`: that is decided by the type of
+    /// `chrome_animation`, so a build without the fade does not carry a flag
+    /// saying so.
     animate_theme_changes: bool = false,
     /// Native pixel attachments supported by the shell (Kitty graphics in a
     /// terminal, GPU textures in SDL). Image panes dynamically fall back to