Remove MMCE one-at-a-time blocking throttle in art loader

[Copilot]

MMCE art almost never popped in and navigation felt clogged. The root cause: a blocking guard in cacheGetTextureInternal prevented BG and SCR from queuing until COV fully finished loading — a slow or missing COV (open() failure on MMCE can take 100–500 ms) stalled all subsequent art types, and an extra render-frame dead gap separated each type.

Changes

• Removed the cacheHasActiveInteractiveModeLocked + cacheHasQueuedInteractiveModeLocked blocking guard from the MMCE throttle block in cacheGetTextureInternal

// Before — BG/SCR blocked until COV fully loaded:
if ((cacheHasActiveInteractiveModeLocked(MMCE_MODE) && !gArtCurrentReq->abortRequested) ||
    cacheHasQueuedInteractiveModeLocked(MMCE_MODE)) {
    cacheUnlock();
    return NULL;
}

// After — removed; all three art types pre-queue together

• Kept the game-change drop (drop a stale queued request when the game changes) as a safety net, with an explanatory comment
• The 1-frame COV head-start delay (from prior commit) remains, so COV still queues on frame 8 and BG/SCR on frame 9
• The art thread is already single-threaded FIFO — it serializes COV → BG → SCR in queue order with no extra mechanism needed; cacheAdvanceGeneration() on navigation drops all queued requests at once and handles abort/discard via generation tracking