Sub-arc 2 of the UX arc, TUI half. When a window reserves a line-number
gutter, lines with diagnostics get a severity-colored sign glyph (E/W/I/H)
in the gutter's leading column — closing the last deferred Task #23 item.
No protocol/daemon change: the per-line severity is already frontend-side
(the diag store the DiagnosticView already reads).
- `Viewport` gains `gutter_w` so overlays can reach the gutter's leading
column at `cell_origin.col - gutter_w`; the text area is already shifted
past it, so viewport-relative painters stay gutter-agnostic.
- The gutter's number pass now runs *before* the overlays (was after), so
the DiagnosticView can draw its sign into the gutter's blanked leading
column without the number pass erasing it.
- DiagnosticView: with a gutter, draw the severity sign glyph colored by
`underline_color()`; without one, keep the legacy column-0 background
marker (the "fake gutter" that predates a real gutter column). Extracted
to `paint_line_markers` to keep `render` under the line cap.
The number never reaches column 0 (>=1 leading pad by construction), so
sign and number coexist. Diagnostic signs currently ride the line-number
gutter (visible when line numbers are on); a signs-without-numbers mode is
deferred.
Test: gutter_sign_replaces_the_column_marker_when_a_gutter_is_reserved.
Validated: fmt + clippy --all-targets clean both flavors; 1441 lib + 22
diag tests pass. Needs a TUI eyeball before the GPU half.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014TXbAwk27agwhrNNrhLi2U
Levi Neuwirth
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