Narrow wordmark for narrow screens; move the art out of the code

JP drew a 53-cell MeshTerm at the original's height. It gets the same
colouring: the vertical ramp — white, yellow, light red, red, light
black — with every box-drawing character, shadow row and letter bevel
alike, in the light black.

The art moves to meshterm/assets/. It was sitting in meshterm/ui/ among
the modules, which is the wrong place for a file nothing imports; it
still ships inside the package, since the splash reads it at runtime and
an installed venv has no repo to read from. scripts/colour-logo.py paints
a .txt into its .ansi, so the palette is stated once in code instead of
retyped into escape sequences by hand, and re-styling stays a text edit.

Which mark is drawn is a question about the *screen*, not the platform: a
53-column console and a desktop terminal dragged narrow have the same
problem. load_logo(cols) answers it — widest that fits — and
compose_startup asks, since it is the only place that knows the width.
Below the smallest mark it returns nothing at all; no banner beats a torn
one. A test now draws the real wordmark at each platform's width, which
is the gate the splash never had (it is the one screen the gallery
doesn't cover, which is how a 71-cell mark shipped to a 53-cell console).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/colour-logo.py

@@ -0,0 +1,73 @@
+#!/usr/bin/env python3
+"""Colour a plain-text wordmark into the pre-coloured ANSI the splash draws.
+
+The art itself is hand-drawn and lives in ``meshterm/assets`` as a ``.txt``; this paints
+it and writes the matching ``.ansi`` beside it, so the colour scheme is stated once here
+rather than re-typed into every escape sequence by hand.
+
+    python scripts/colour-logo.py                 # repaint every asset/*.txt
+    python scripts/colour-logo.py logo_53         # just that one
+
+The scheme is a vertical ramp over the letter rows — white, yellow, light red, red, light
+black, hottest at the top — with every box-drawing character (the drop shadow and the
+letters' own bevels) in the light black regardless of its row. Each colour is chosen so the
+PicoCalc console's 16-slot quantizer lands it on exactly one palette slot (15, 11, 9, 1, 8)
+while it stays a warm fade in truecolour on the desktop; see ``theme._VT_SLOTS``.
+"""
+
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+ASSETS = Path(__file__).resolve().parent.parent / "meshterm" / "assets"
+
+#: Letter-row colours, hottest first. A mark with more rows than this reuses the last one.
+ROW_RGB = ("255;255;255", "251;217;80", "248;113;113", "185;28;28", "80;93;111")
+
+#: Every non-block character — the drop shadow and the bevels — takes this.
+SHADOW_RGB = "80;93;111"
+
+#: The letter body. Everything else in the art is shadow.
+BLOCK = "█"
+
+
+def colour(art: str) -> str:
+    """Paint one plain-text wordmark, returning the pre-coloured ANSI."""
+    lines = art.split("\n")
+    if lines and lines[-1] == "":
+        lines.pop()
+    out_lines = []
+    for row, line in enumerate(lines):
+        hue = ROW_RGB[min(row, len(ROW_RGB) - 1)]
+        out, current = [], None
+        for ch in line:
+            if ch == " ":
+                if current is not None:
+                    out.append("\x1b[0m")
+                    current = None
+                out.append(" ")
+                continue
+            want = hue if ch == BLOCK else SHADOW_RGB
+            if want != current:
+                out.append(f"\x1b[38;2;{want}m")
+                current = want
+            out.append(ch)
+        if current is not None:
+            out.append("\x1b[0m")
+        out_lines.append("".join(out))
+    return "\n".join(out_lines) + "\n"
+
+
+def main() -> None:
+    """Repaint the named marks, or every ``.txt`` in the assets folder."""
+    names = sys.argv[1:] or sorted(p.stem for p in ASSETS.glob("*.txt"))
+    for name in names:
+        source = ASSETS / f"{name}.txt"
+        target = ASSETS / f"{name}.ansi"
+        target.write_text(colour(source.read_text(encoding="utf-8")), encoding="utf-8")
+        print(f"{source.name} -> {target.name}")
+
+
+if __name__ == "__main__":
+    main()