Diagnostics shows how big TuiCode thinks a terminal cell is

[a-team-app]

Context

The Retina bug in #301 went unnoticed for a release because nothing shows how big TuiCode thinks a terminal cell is. This slice adds that to the diagnostics overlay, alongside the cursor colour row, so a wrong measurement is one keystroke away on the machine that's wrong. Pitch: #301. Needs #333 merged first: the row shows the scale that task reads.

Acceptance criteria

• The diagnostics overlay (F12, or mnemonic sd) has a Cell size row, shown as a Label pair like the rows around it.
• In iTerm2 on a Retina screen it reads like 16 × 35 px • iTerm2 report, scale 2.0.
• In iTerm2 on a non-Retina screen it shows scale 1.0.
• In a terminal that answers the other query it reads like 10 × 20 px • CSI 16 t.
• Where no terminal answer arrived it reads 10 × 20 px • no answer, assumed.
• The row fits on one line and doesn't push other rows out of the overlay at its usual size.

Tests

• The row's text for each source: iTerm2 report with its scale, CSI 16 t, and the assumed default.
• The source reported is the one whose answer was actually used ("first answer wins").

Out of scope

• Re-probing from the overlay, or a button to do so.
• Showing both queries' answers, or timings.
• Any change to how the cell size is detected.

[a-team-app]

Blocked by #333: The Cell size row shows the scale factor that #333 starts reading.

[a-team-app]

Draft PR #352 is up.