protect_first_n decay state (compression_count / _previous_summary) is
in-memory only, so a gateway restart re-protected the persisted handoff
summary and head fossils, growing the head unboundedly across
restart+compaction cycles (#57814).
_effective_protect_first_n now probes a bounded resumed-head window for
a persisted handoff summary (by metadata or content prefix) and decays
protection when one is found, before compress_start is computed. The
first post-restart compaction self-heals: stacked summary fossils are
folded into the next summary prompt instead of preserved verbatim,
rehydration is rolled back on abort, deterministic fallback carries a
bounded redacted previous-summary snapshot, and forced user-leading
merged summaries keep the live tail request after the summary end
marker so they stay rehydratable.
Squashed reapply of the 12-commit series from PR #57835 onto current
main (branch was 1210 commits behind; single add/add conflict with the
task-snapshot grounding helpers resolved by keeping both).
Fixes#57814.
harjoth
08ea88f4f7