The GPU's gutter-sign equivalent (framing Q#D3). The producer folds
diagnostics into the per-line summary: each touched line's dominant
style gets the severity's canonical underline_color (most severe
wins), skipped while the URI's store entry is stale — same
discipline as the decorations producer. The minimap stroke prefers
underline_color over the syntax fg, so error/warning lines read at
a glance.
Diagnostics publish without a CRDT generation bump, so the
summary's generation-keyed cache gains a second key: a new per-URI
epoch on DiagnosticStore (bumped on set/clear, not mark_stale). A
republish re-emits the summary; everything else stays suppressed
(framing bet #3 — the gate widens precisely, not naively).
DiagnosticSeverity::underline_color() becomes the canonical palette
(TUI squiggles, col-0 markers, and minimap marks all share it).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Levi Neuwirth
c2f6905b08