fix(plymouth): #137 splash follows the canvas; #145 keyboard hint on the LUKS prompt

The logo was off-centre on the external when booting/shutting down docked.
Both earlier diagnoses in this repo were WRONG, and that is the part worth
keeping: Window.GetWidth() returns max_width (not head 0), and per-head
sprites would have DUPLICATED the logo — the script plugin already builds
one max_width x max_height canvas, centres every display inside it
(script-lib-sprite.c:536) and draws sprites at (sprite.x - display.x), so a
canvas-centred sprite is centred on every head. The arithmetic was right.

The bug was TIME: every position was a top-level statement evaluated once
at parse time. A head arriving/leaving mid-splash resizes the canvas, the
plugin re-centres the displays, and frozen sprites end up off by
(new_max - old_max)/2 on every head. One monitor never resizes the canvas —
hence "only when docked". Fix: one layout(), re-run from the existing
refresh callback on canvas change.

The trap that cost two attempts (now a comment): in plymouth script a bare
assignment inside a function writes the GLOBAL if that name already exists
globally. So `canvas_width = Window.GetWidth()` updated global.canvas_width
BEFORE the guard compared against it — always false, body never ran, splash
rendered as a bare background, NO error logged. Isolated with a 20-line
probe. Hence cw/ch.

#145 rides along: a VT loads one keymap and knows nothing of per-device
layouts, so the passphrase box types with the console layout — worth saying
before three wrong tries on a disk you cannot read (same gap as #114, one
step earlier). Fedora's mechanism does not port: ply_keymap_icon is a C
widget in the two-step plugin (the script plugin has no keyboard API), fed
from XKBLAYOUT in /etc/vconsole.conf, which NixOS never writes (plymouth's
trace says `XKBLAYOUT: (null)`). So @LAYOUT@ is baked from
services.xserver.xkb.layout like the palette — the FIRST of a comma list,
since that is what the VT loads. Icon is plymouth's own keyboard.png (the
same glyph Fedora shows), copied at build time so no GPL bytes enter the
repo, recoloured to subtext.

chmod +w after that copy: store files are 444 and recolor rewrites in
place. Without it magick fails, the phase aborts, and EVERY LATER SED
SILENTLY DOES NOT RUN — the theme shipped with @BG_R@ literals and I only
caught it by reading the build log.

Verified by render (tools/plymouth-preview.sh, real built theme): both
heads centred, password dialog showing padlock + entry + keyboard icon and
"us". V3 queued — only a real docked boot can resize a canvas.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/nixos/plymouth.nix

@@ -24,8 +24,20 @@ let
   text = colorOf "text" "#D3DAE0";
   surface = colorOf "surface" "#303A46";
   accent = colorOf "accent" "#B79BE8";
+  # The keyboard hint is a footnote, not a headline: subtext, like the rest of
+  # the palette's secondary text (#145).
+  subtext = colorOf "subtext" "#97A3B2";
   magick = lib.getExe' pkgs.imagemagick "magick";
 
+  # What the passphrase prompt actually types with. `services.xserver.xkb.layout`
+  # is this distro's single source for the layout, bridged to the console (and
+  # so to the initrd prompt) by console.useXkbConfig — see modules/nixos/default.nix.
+  # A VT loads exactly ONE keymap, so a comma list ("us,gb") means the FIRST:
+  # printing the raw string would lie in precisely the multi-layout case the
+  # label exists for.
+  kbdLayout = lib.head
+    (lib.splitString "," (config.services.xserver.xkb.layout or "us"));
+
   # Plymouth's Window.SetBackgroundTopColor takes three floats in
   # 0.0–1.0; the .plymouth metadata's ConsoleLogBackgroundColor takes a
   # 0xRRGGBB hex. Nix has no float math: multiply, integer-divide, pad.
@@ -38,7 +50,9 @@ let
         else if lib.stringLength s == 2 then "0${s}"
         else s;
     in "0.${padded}";
-  channel = off: byteToFloat (lib.fromHexString (lib.substring off 2 base));
+  channelOf = hex: off: byteToFloat (lib.fromHexString (lib.substring off 2 hex));
+  channel = channelOf base;      # Window.SetBackground* (the splash base)
+  fgChannel = channelOf subtext; # Image.Text needs the same 0.0-1.0 floats
 
   nomarchy-plymouth = pkgs.stdenv.mkDerivation {
     pname = "nomarchy-plymouth";
@@ -53,6 +67,17 @@ let
       mkdir -p "$themedir"
       cp * "$themedir/"
 
+      # The keyboard glyph for the passphrase hint (#145) is plymouth's own —
+      # literally the icon Fedora shows, since its keymap widget loads this same
+      # asset. Copied at build time rather than vendored: the bytes stay in
+      # nixpkgs' (GPL) plymouth and never enter this repo, and it cannot drift
+      # from the plymouth we actually run.
+      # chmod: store files are read-only (444) and `recolor` rewrites in place.
+      # unpackPhase makes the *source* writable, which is why the art below
+      # needs no such thing — a file copied straight from the store does.
+      cp ${pkgs.plymouth}/share/plymouth/themes/spinner/keyboard.png "$themedir/keyboard.png"
+      chmod +w "$themedir/keyboard.png"
+
       # Recolor the splash art from the palette (flat fill, alpha kept) so
       # it reads on any base instead of the shipped fixed navy.
       recolor() { ${magick} "$themedir/$1" -fill "#$2" -colorize 100 "$themedir/$1"; }
@@ -62,6 +87,7 @@ let
       recolor entry.png        ${surface}  # password field box
       recolor progress_box.png ${surface}  # progress track
       recolor progress_bar.png ${accent}   # progress fill
+      recolor keyboard.png     ${subtext} # passphrase keyboard-layout hint
 
       # Point the .plymouth metadata into the store
       sed -i "s|/usr/share/plymouth/themes/nomarchy|$themedir|g" \
@@ -73,6 +99,10 @@ let
         -e 's|@BG_R@|${channel 0}|g' \
         -e 's|@BG_G@|${channel 2}|g' \
         -e 's|@BG_B@|${channel 4}|g' \
+        -e 's|@FG_R@|${fgChannel 0}|g' \
+        -e 's|@FG_G@|${fgChannel 2}|g' \
+        -e 's|@FG_B@|${fgChannel 4}|g' \
+        -e 's|@LAYOUT@|${kbdLayout}|g' \
         "$themedir/nomarchy.script"
       sed -i 's|@BG_HEX@|${base}|g' \
         "$themedir/nomarchy.plymouth"