Check that a thinned tick row still ends where the range does

fitMarks picks a stride that divides the interval count, so the last label
lands on the end rather than short of it. Five marks cannot reach that guard
— find() settles on a divisor first — so the check carries a six-mark case
that does.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K1moruzue2kTJd3uVisgNk

frontend/src/components/Dashboard/ui/core/controls.check.ts

@@ -0,0 +1,60 @@
+/**
+ * The tick-label thinning, checked.
+ *
+ * ponytail: a script rather than a suite, matching `color.check.ts` — pure
+ * arithmetic needs no browser:
+ *
+ *     cd frontend && bun run src/components/Dashboard/ui/core/controls.check.ts
+ */
+import assert from "node:assert/strict"
+import { fitMarks } from "./controls"
+
+const marks = (...labels: string[]) => labels.map((label) => ({ label }))
+const labels = (kept: { label: string }[]) => kept.map((m) => m.label)
+
+const five = marks("20", "20.5", "21", "21.5", "22")
+
+// Unmeasured, so nothing is thinned yet.
+assert.deepEqual(fitMarks(five, 0), five)
+
+// Two labels are the ends themselves and can never crowd.
+assert.deepEqual(fitMarks(marks("0", "1"), 1), marks("0", "1"))
+
+// The default four columns keeps all five.
+assert.deepEqual(labels(fitMarks(five, 261)), [
+  "20",
+  "20.5",
+  "21",
+  "21.5",
+  "22",
+])
+
+// Three columns thins to the whole numbers, ends included.
+assert.deepEqual(labels(fitMarks(five, 177)), ["20", "21", "22"])
+
+// Across every width: the ends survive, and the stride divides the intervals
+// so the last label lands on the end rather than short of it. Four intervals
+// may thin by 1, 2 or 4 — never 3.
+for (let width = 1; width <= 600; width++) {
+  const kept = labels(fitMarks(five, width))
+  assert.equal(kept[0], "20", `first lost at ${width}px`)
+  assert.equal(kept[kept.length - 1], "22", `last lost at ${width}px`)
+  assert.ok(
+    [2, 3, 5].includes(kept.length),
+    `${kept.length} labels at ${width}px`,
+  )
+}
+
+// Six marks reach the divisor guard that five cannot: at this width the
+// narrowest stride that fits is 3, which does not divide five intervals and
+// would drop the last label. It must fall through to 5 and keep both ends.
+const six = marks("0", "1", "2", "3", "4", "5")
+assert.deepEqual(labels(fitMarks(six, 60)), ["0", "5"])
+
+// Wider labels thin sooner than narrow ones at the same width.
+assert.ok(
+  fitMarks(marks("1000.5", "1001", "1001.5"), 120).length <=
+    fitMarks(marks("1", "2", "3"), 120).length,
+)
+
+console.log("fitMarks: ok")