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fix(signals): refuse flush() inside an action body (FLUSH_IN_ACTION)

An action's writes are held by its transaction until it settles, so a
flush() in the body can't reveal them. Worse, the drain loop exits only
once activeTransition is null, so it parked the transaction mid-slice and
every write after it landed as a plain committed write — visible before
the action finished. That is what the 'leading flush()' workaround in
#3333 was actually doing.

- actionStepDepth counter brackets it.next()/it.throw() in step()
- flush(): DEV throws FLUSH_IN_ACTION; prod skips the drain (flush(fn)
  still runs fn, its writes stay in the transaction)
- restoreTransition skips its own drain while a step is on the stack: a
  nested action resuming synchronously (non-thenable yield) inside an
  outer body had the same leak
- tier-aware tests (SIGNALS_TIER=prod exercises the no-op path)

Closes #3333

Co-authored-by: Claude via Cursor <noreply@cursor.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

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