fix(design): badge text clears AA — the ladder was painting a hue on a tint of itself (#3132)

Every status and priority badge used its raw hue as TEXT on a 12% tint of
that same hue. Measured on the dark palette, all six pairs failed the kit's
own AA floor: todo 1.60:1, in-progress 1.97:1, done 2.06:1, low 2.02:1,
high 2.92:1, medium 2.97:1, against 4.5. Four also failed in light mode.

The cause is structural, not a bad colour pick. A 12% tint sits near the
surface it composites over, so the hue as text on it has almost nowhere to
go. Strengthening the tint was measured and REJECTED: on a dark palette a
heavier tint moves the chip toward the light text and makes it worse. 12%
was already optimal.

So each pair gains a `-fg` sibling: the hue mixed toward --fs-text-primary
until it clears 4.5:1 worst-case over surface-raised AND surface-hover in
BOTH modes. Mixing toward that token rather than a literal is what makes one
declaration cover both — it inverts by mode, so the text follows.

Recorded in the DESIGN SYSTEM, not hand-written into theme.css: seven tokens
on design system 2, each carrying its measurement and its reasoning, then the
sheet regenerated. theme.css says not to hand-edit the --fs-* block and it is
right — a hand-edit would be silently reverted by the next regeneration.

The ladder keeps its shape. High priority still holds 52% saturation and
medium 31% — the rungs that need to shout still shout. Low, todo and done
wash toward neutral, which is what their own rationales ask for: status-todo
is derived from the border colour precisely so not-yet-started recedes.
Receding and illegible are different things and the old value was the second.

--fs-status-cancelled-fg was found by measuring, not by reasoning. Cancelled
derives from --fs-text-tertiary, which looks like the obviously-correct
"quiet" choice and is a HINT colour tuned for plain surfaces — 2.63:1 on a
badge tint in light mode.

StatusBadge additionally dropped a `color-mix(..., #000 15%)` that darkened
the hue: a light-mode instinct that made these worse on a near-black surface,
and a literal besides.

THE GUARD IS THE POINT. check_design_tokens.py now FAILS on any rule that
paints text with a token on a tint of that same token, and names the -fg
sibling as the fix. Verified by reintroducing the defect: exit 1 with it,
exit 0 without. Unlike a raw literal there is nothing to weigh up, so it
gates rather than reports.

Two `border-top-color` uses keep the raw hue, correctly — a border is a
non-text graphic and needs 3:1, which is what the hue is for.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/assets/theme.css

@@ -12,6 +12,13 @@
    file used to read, and it is deliberate: the light palette was never specified
    by any rule, so it is recorded as a departure rather than as the default.
 
+   The -fg tokens are a badge's TEXT colour, added because the ladder used its
+   raw hue as text on a 12% tint of the same hue — measured 1.60-2.97:1 on the
+   dark palette against the kit's AA floor of 4.5. Each is the hue mixed toward
+   --fs-text-primary until it clears 4.5:1 worst-case over surface-raised and
+   surface-hover in BOTH modes. Mixing toward that token is what makes one
+   declaration cover both: it inverts, so the text follows the mode.
+
    Only 12 tokens differ between modes. Everything else — spacing, type, motion,
    radius, and every derived colour — is stated once, because a value built with
    var() resolves where it is USED, not where it is written.
@@ -78,10 +85,13 @@
   /* priority */
   --fs-priority-low: var(--fs-info);
   --fs-priority-low-bg: color-mix(in srgb, var(--fs-priority-low) 12%, transparent);
+  --fs-priority-low-fg: color-mix(in srgb, var(--fs-priority-low) 45%, var(--fs-text-primary));  /* Badge TEXT for low priority — the readable partner of the -bg tint */
   --fs-priority-medium: var(--fs-warning);
   --fs-priority-medium-bg: color-mix(in srgb, var(--fs-priority-medium) 12%, transparent);
+  --fs-priority-medium-fg: color-mix(in srgb, var(--fs-priority-medium) 55%, var(--fs-text-primary));  /* Badge TEXT for medium priority */
   --fs-priority-high: var(--fs-error);
   --fs-priority-high-bg: color-mix(in srgb, var(--fs-priority-high) 12%, transparent);
+  --fs-priority-high-fg: color-mix(in srgb, var(--fs-priority-high) 55%, var(--fs-text-primary));  /* Badge TEXT for high priority */
 
   /* radius */
   --fs-radius-sm: 4px;  /* pills, tags, code spans */
@@ -116,12 +126,16 @@
   /* status */
   --fs-status-todo: var(--fs-border-color);
   --fs-status-todo-bg: color-mix(in srgb, var(--fs-status-todo) 12%, transparent);
+  --fs-status-todo-fg: color-mix(in srgb, var(--fs-status-todo) 40%, var(--fs-text-primary));  /* Badge TEXT for a not-started task */
   --fs-status-in-progress: var(--fs-accent);
   --fs-status-in-progress-bg: color-mix(in srgb, var(--fs-status-in-progress) 12%, transparent);
+  --fs-status-in-progress-fg: color-mix(in srgb, var(--fs-status-in-progress) 45%, var(--fs-text-primary));  /* Badge TEXT for a task underway */
   --fs-status-done: var(--fs-success);
   --fs-status-done-bg: color-mix(in srgb, var(--fs-status-done) 12%, transparent);
+  --fs-status-done-fg: color-mix(in srgb, var(--fs-status-done) 50%, var(--fs-text-primary));  /* Badge TEXT for a completed task */
   --fs-overdue: var(--fs-error);
   --fs-status-cancelled: var(--fs-text-tertiary);  /* set aside, not failed */
+  --fs-status-cancelled-fg: color-mix(in srgb, var(--fs-status-cancelled) 60%, var(--fs-text-primary));  /* Badge TEXT for a cancelled task */
 
   /* surface */
   --fs-surface-page: #14171A;  /* page bg, deepest surface */