Fix "Stripes" preset span positions collapsing on repeated updateStops calls

[Copilot]

The hadLinked heuristic in updateStops incorrectly treated explicit span end-positions (e.g. 0% 20%) as auto-linked pairs on any call after the first, collapsing them to 0% 0%. This rendered the Stripes preset as solid black after the first user interaction with the overlay.

Root cause

After the initial updateStops pass, stop.auto is set to the computed auto value (e.g. 20 for a stop at index 2). On the next call, position2 = '20' matched prevAuto = 20, triggering hadLinked = true and overwriting position2 ← position1 = '0':

// Before fix: prevAuto condition fires regardless of whether p1 is auto-managed
const hadLinked = (!p2Unset) && (
  String(stop.position2) === String(stop.position1) ||
  (prevAuto != null && String(stop.position2) === String(prevAuto))  // ← false positive
)

Fix

Guard the prevAuto branch with p1Unset — a stop's position2 should only be considered "linked to auto" when position1 is also auto-managed:

const hadLinked = (!p2Unset) && (
  String(stop.position2) === String(stop.position1) ||
  (p1Unset && prevAuto != null && String(stop.position2) === String(prevAuto))
)

This preserves explicit spans like #000 0% 20% across repeated updateStops calls while keeping the redistribution linkage intact for fully auto-managed stop pairs.

Tests

Added src/lib/stripesIssue.test.ts covering idempotency of span positions across multiple updateStops calls and correct CSS output after re-normalization.

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