docs: mode_style spells thirteen variants, not fourteen

`context/code/tui.md` claimed `mode_style`'s default-chrome arm spells out
fourteen `AppMode` variants. It spells thirteen — Normal, RecordsNormal,
FormulaEdit, FormulaPanel, CategoryPanel, CategoryAdd, ItemAdd, ViewPanel,
ImportWizard, ExportPrompt, SavePrompt, Help, Quit — counted off `src/draw.rs`
rather than taken on report.

The match itself is sound and exhaustive; only the sentence about it was wrong.
The arithmetic that makes the corrected count checkable now sits beside it:
13 + Editing/RecordsEditing + CommandMode + TileSelect = 17, which is
`AppMode`'s variant count.

Nothing in this repository checks a count written in prose, which is exactly why
it survived — the same failure mode gotcha 38's derivation-count correction and
gotcha 44's stale gap list record.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH

context/code/tui.md

@@ -122,9 +122,13 @@ enum, and only the last translation is local.**
 `mode_style(&AppMode) -> Style` stays here for the same reason (`Style` and
 `Color` are ratatui types and `improvise-command`, where `AppMode` lives,
 carries no rendering-framework dependency), and since `improvise-zx8t` it is
-**exhaustive**: the fourteen variants that take the default chrome are spelled
+**exhaustive**: the thirteen variants that take the default chrome are spelled
 out rather than swept up by a `_ =>` wildcard, so adding an `AppMode` fails the
-build here until someone chooses its colour.
+build here until someone chooses its colour. The arithmetic is the check a
+prose count does not get: 13 + `Editing`/`RecordsEditing` + `CommandMode` +
+`TileSelect` = 17, which is `AppMode`'s variant count. It read *fourteen* until
+2026-09-22, and nothing could have caught that — a count written in prose is
+checked by no one.
 
 It is still not quite the same shape as `axis_fg` and its two siblings, and the
 difference is deliberate rather than pending. Those three map a *semantic enum*