Fix Stripes preset: strict equality for auto-position suppression, suppress auto-computed hints

[Copilot]

The Stripes (and Neon Stripe) gradient presets generated incorrect CSS output — missing span end positions (#000 0% instead of #000 0% 20%) and including noisy auto-computed midpoint hints — causing stripes to render as solid black.

Root causes

• Loose equality (==) for auto-position suppression — String(p1) == String(s.auto) coerced the explicit string '0' (from preset position1: '0') to equal the auto-computed number 0, stripping the start of spans. #0ff 0% 12% became #0ff 12%.
• Auto-assigned 0% leaked onto the first stop — a no-position leading stop (e.g., {color: '#fff'}) got position1=0, position2=0 from updateStops; only p1 was suppressed, leaving an unwanted trailing 0%.
• isPctZero stripped the 0% span start — no guard for the presence of p2, so a stop like {position1: '0', position2: '12'} had its 0% incorrectly removed.
• Gradient.svelte included auto-computed hints — the local stopsToStrings was missing the same midpoint-hint suppression that gradientString.ts already had.

Changes

• src/utils/gradientString.ts: p1 === s.auto (strict) replaces String(p1) == String(s.auto); isPctZero only strips when no p2 exists; suppresses auto-assigned p2 on the first stop when p1 was also auto-suppressed.
• src/components/Gradient.svelte (stopsToStrings): Same strict-equality fix and first-stop p2 suppression; adds s.percentage == s.auto guard to skip auto-computed midpoint hints.

Before / after for the Stripes preset:

/* before */
linear-gradient(to top right in oklab, #fff, #000 0%, #fff 0%, #000 0%, #fff 0%, #000 0%)

/* after */
linear-gradient(to top right in oklab, #fff, #000 0% 20%, #fff 0% 40%, #000 0% 60%, #fff 0% 80%, #000 0% 100%)

• src/lib/roundtrip.test.ts: Two new test cases using stopWithAuto/hintWithAuto helpers that simulate real updateStops output — covering the clean first-stop case and the preserved explicit span case.

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