Two comment-only corrections from the final review round.
The `isCompactCheckpoint` redundancy note appealed to the mandatory-marker
invariant to explain something the enclosing branch now states outright:
after e029ffb88 both call sites are literally inside an
`isReplacementSurfaceEvent` branch, so the appeal became retained
reasoning. Say the call sites test it directly.
`appendPreCompactionLog` implied its tool-result content is what survives
compaction, but `bash`'s presenter is static, so the string never reaches
a fixture — the fixtures pin that the shadowed step's card survives.
Neutral text plus a note on where the card body comes from, so a later
reader does not "fix" a fixture to make the sentence true. Verified by the
absence of fixture drift from changing the string.