fill the diff before draining buffered head updates

[cheggaaa]

Follow-up to #772.

FillDiff bulk-Sets the diff from a storage snapshot. With the drain goroutine started first, a RemoveId the drain had just applied got undone by that older snapshot — resurrecting a deleted tree in the diff and persisting a space hash for a tree the space no longer has. Nothing corrected it afterwards, so head sync kept advertising the tree until the next unrelated update for that id.

The race predates #772, but #772 made it reachable: subscribing from Init means updates buffer for the whole Run phase (~13 components) instead of being dropped, so the drain flushes a burst at exactly the moment FillDiff runs. Previously the window was microseconds and nothing was pending.

Filling first makes the storage snapshot the baseline and lets buffered updates apply on top, where they converge — the last queued event for an id always matches storage. The reverse order could not converge, because the bulk Set had no later event to correct it.

FillDiff stays in Run: it is ctx-bound I/O that can fail, and Init has no context. Only the two statements swap — no component lifecycle changes.

go test -race ./... green (58 packages).

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https://claude.ai/code/session_01HXNxMzxa6rK3bAwcULTt4x

[github-actions]

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