InitialState

The InitialStateContainer is a lookaside table used by a ComponentTree to create initial states for Components. The idea is that the onCreateInitialState result for each component will be cached and stored here so that we can guarantee it's not called multiple times on multiple threads. We keep the initial states cached as long as there is a layout happening for the ComponentTree. As soon as we detect that all in-flights layout have terminated we can clean up the initial states cache.

Locking discipline, as numbered invariants:

• INV-1 (single monitor): every read or write of a Slot's contents happens inside synchronized(slot.lock) for the slot returned by this invocation's slotFor.

• INV-2 (fetch-once): each public method calls slotFor exactly once, holds the result in a local, and never re-reads slots after acquiring the lock. Without this, a drain between the vend and the lock acquisition would let two threads hold different monitors over the same logical key.

• INV-3 (co-drain): monitor and data are allocated together (Slot) and discarded together (slots = null), so a drain cannot go partial. Any future per-key cache belongs inside Slot, never beside it. A thread still holding a pre-drain Slot keeps writing to it and its seed is discarded rather than resurrected into the new epoch -- safe only because a registered StateHandler keeps pendingStateHandlers non-empty and so prevents the drain.