fix: measure terminal columns in cells so CJK text lands correctly - #3

Problem

Typing Korean (or any East Asian text) into the annotation editor puts the cursor in the wrong place, and the character being composed appears to duplicate itself inside the text that is already there.

Both symptoms come from one line. commentLines() in src/editor.ts advances the column by one for every character:

lines[row] += char;
col += 1;

East Asian characters occupy two terminal cells. After typing five Hangul syllables the text is ten cells wide, but the editor reports column five and places the cursor there with \x1b[{row};{col}H. The cursor sits in the middle of the text.

The apparent duplication is the same bug seen through the IME. While a syllable is being composed the terminal paints the preedit overlay at the cursor position the application reported, which is mid-text, so the composing syllable shows up inside what was already typed. On commit the application redraws and the syllable jumps to the end.

Fix

Add src/width.ts, which reports how many cells a character occupies: two for Hangul syllables and jamo, CJK ideographs, kana, fullwidth forms, and emoji, one for the rest, zero for control characters and combining marks.

Measure every column in cells rather than characters:

• src/layout.ts (extracted from editor.ts) computes the cursor row and column in cells, and wraps a line before a wide character would straddle the right edge.
• wrapText() in src/format.ts wraps on cells, so Korean selections no longer overflow the box.
• The editor footer and the manager's list entries clip on cells.
• Vertical cursor motion keeps the visual column through mixed-width text.

commentLines() moved into its own module because editor.ts starts the TUI as an import side effect, which leaves the geometry untestable where it was.

Tests

15 new tests covering cell widths, comment layout, and wrapping. The full suite is 53 tests, all passing, and bun run typecheck is clean.

bun test    53 pass, 0 fail

Reported and reproduced on macOS with a Korean IME.