fix(design/variants): guard --count against NaN/zero/negative input

[jbetala7]

Problem

design variants --count <bad value> silently produces zero variants and emits a misleading JSON result.

--count is parsed with parseInt and never validated (design/src/cli.ts:200), and variants() only clamped the upper bound (design/src/variants.ts):

const count = Math.min(options.count, 7); // Cap at 7 style variations

So bad inputs flow through. Measured against the real clamp + JSON-emit path on main (c7ae632):

--count	generated	JSON count
abc	0	null
0	0	0
-2	0	-2
3	3	3
10	7	7

Math.min(NaN, 7) is NaN, so for (i = 0; i < NaN; i++) never runs and the command exits "successfully" having created nothing, reporting "count": null. A single typo (--count e) is enough.

Root cause

No lower-bound or NaN guard on the requested count — only the <= 7 cap.

Fix

Add a small exported, pure normalizeVariantCount helper and use it in place of the bare cap:

export function normalizeVariantCount(count: number): number {
  if (!Number.isFinite(count)) return 3;            // unparseable -> default
  return Math.max(1, Math.min(Math.trunc(count), 7)); // clamp to [1, 7]
}

• non-finite (NaN/Infinity) -> default 3
• 0 / negative -> 1
• > 7 -> 7 (unchanged)
• in-range -> unchanged

This mirrors the exported-helper validation shape proposed for --api-timeout in #1528, but is independent of it (different flag).

Testing

bun test design/test/variants-retry-after.test.ts -> 10 pass / 0 fail (5 existing + 5 new). Full bun test design/test/ passes. The helper is pure, so the regression is pinned without an ANTHROPIC_API_KEY/OPENAI_API_KEY.

Scope / collision check

• Touches only design/src/variants.ts + its test. No behavior change for valid counts.
• Open design PRs are about timeouts and unrelated concerns (fix(design/variants): surface real 240s timeout in AbortError message #1774 timeout label, design: bump per-image OpenAI timeout from 120s to 240s #1638 240s bump, fix(design): add --api-timeout flag, raise default 120s→300s for image-gen #1528 --api-timeout guard, fix: preserve every generated design variant in plan-design-review #1737 variant preservation) — none guard --count. Searched variants, design count, design timeout; no overlap.

Fixes #2032

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[time-attack]

@16francej ensures numers are real. merge