Measure the terminal cell instead of guessing it, and stop telling the PTY zero

TERMINAL_CELL_WIDTH was 8.4 — a number nobody had measured. Asking the text
system for the advance of `0` in the face actually being painted gives ~7.3, so
the guess was 13% wide and the terminal was losing eighteen columns: the same
window that reported 122 columns now reports 140. Layout and paint now read the
same measurement, so they cannot drift apart again.

The plan claimed ws_xpixel disagreed with the painted width by 0.4 px per
column. It did not: the app only ever called TerminalSize::new, which passes no
pixel dimensions, so ws_xpixel and ws_ypixel were both *zero*. Every program
doing pixel arithmetic — sixel, the kitty graphics protocol, anything sizing an
image to the viewport — was being told the window has no size at all. Both the
spawn and the resize paths now report the real extent.

TerminalDimensions gains cell_width/cell_height accessors: it was already
carrying the values and nothing could read them.

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

crates/lumbridge-terminal/src/lib.rs

@@ -66,6 +66,23 @@ impl TerminalDimensions {
         self.rows
     }
 
+    /// The measured width of one cell, in pixels.
+    ///
+    /// Carried so a caller can report the viewport's pixel extent to a PTY.
+    /// `ws_xpixel` is the whole window, not one cell, so it is this times the
+    /// column count — and a terminal that reports zero there tells every program
+    /// doing pixel arithmetic that the window has no size.
+    #[must_use]
+    pub const fn cell_width(self) -> u16 {
+        self.cell_width
+    }
+
+    /// The height of one row, in pixels.
+    #[must_use]
+    pub const fn cell_height(self) -> u16 {
+        self.cell_height
+    }
+
     #[must_use]
     pub const fn columns(self) -> u16 {
         self.columns