feat: auto-preload selectPlacements for configured accounts

[mattbodle]

Summary

Adds account-scoped preload configuration to the Rokt Kit. When a partner's account ID is listed in PRELOAD_CONFIGS and all required attributes for that account are present, the Kit automatically calls selectPlacements with preload: true. WSDK caches the experiences response so the confirmation page can render without a second API round-trip.

Ulta configuration (account 2550745407543340151):

• Required attributes: email, firstname, cartitems
• Preload identifier: RoktExperience
• omitUrl: true — makes the cache key page-independent

Trigger points: The check runs after every setUserAttribute, onUserIdentified, processEvent, and when the launcher finishes initializing, so the preload fires as soon as the complete attribute set is available regardless of ordering.

Session gate: self.hasPreloaded ensures the preload fires at most once per tab/session, even if attributes are updated again afterward.

Testing Plan

• 6 new unit tests in test/src/tests.js covering: unknown account no-op, partial attributes no-op, trigger via setUserAttribute, trigger via onUserIdentified, once-per-session guard, and uninitialized-kit guard.
• All 133 tests pass in Chrome and Firefox.
• End-to-end validation requires the WSDK PR (sdk-web#1184) to be deployed — the preload: true flag on selectPlacements is handled there.