Preempt to a higher-priority account when one is available

[MagicalTux]

Follow-up refinement to the priority feature from #28.

Problem

priority was only consulted inside _selectNext, which runs only when a switch is forced (current account exhausted/throttled, or on requalify/session-reset). getActiveAccount returns the current account as long as it's available — without checking priority. So if rotation landed on a lower-priority account and your preferred (higher-priority) account recovered, the proxy wouldn't resume it until the current one ran low.

Change

getActiveAccount now preempts a healthy current account when a strictly higher-priority (lower value) account is available, while staying sticky within the same priority tier:

if (this._isAvailable(current)) {
  const betterExists = this.accounts.some(a =>
    this._isAvailable(a) && (a.priority || 0) < (current.priority || 0));
  return betterExists ? this._selectNext() : current;
}

Why this is safe

• Common case unchanged: with all accounts at the default priority 0, no account is strictly higher than another, so betterExists is always false and behavior is identical to today — no extra switching, no thrash.
• Sticky within a tier: equal-priority accounts never preempt each other, so the per-request weekly-reset heuristic doesn't cause churn.
• Exhaustion + weekly heuristic intact: _isAvailable (session/weekly threshold) is still checked first, and _selectNext still applies the soonest-weekly-reset tiebreaker within a tier.

Tests

test/rotation-priority.test.js extended: preemption to a higher tier, stickiness within a tier, and the all-equal-priority no-op. 8/8 passing.

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