Cold children consult the optional session-projection-cache checkpoint
before paying a preparation recompute: the identity is immutable once
appended, so a cached value is definitive regardless of its watermark.
The cache stays a read-only accelerator (absent service or any rung-two
fault falls through silently; verdicts stay with the authoritative
refold), and the note plus core-data-structures pages describe the
three-rung ladder.
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