V0.5+: compact memory pruning in continuous fold (daemon settled + iOS reducer)

[yyq1025]

Context

V0 maintains each session's cold-path snapshot (SessionRuntime.settled) via a continuous in-memory fold of EventDeltas (packages/daemon/src/messages/fold.ts, foldEventDelta). This is the daemon-side mirror of the iOS transcript reducer (transcript-collection-factory onEvent) — both apply the same deltas the same way, so the daemon's cold snapshot equals what iOS shows live, with no JSONL re-read (no dependency on the SDK's async flush timing, no live/settled divergence).

The deferred bit

compact_started / compact_applied are a deliberate no-op on both sides in V0:

• iOS reducer no-ops compact (divider + prune deferred).
• foldEventDelta mirrors that no-op so settled == live.

Re-reading the compacted JSONL on compact_applied would instead prune settled (filter to the post-compaction surviving UUIDs) and thus diverge from iOS's still-unpruned live state — which is why we don't do it today.

Consequence: after a compaction, both the daemon's settled and the iOS transcript keep the full pre-compaction timeline in memory (correct for rendering, but not pruned). No divider is shown.

V0.5+ work

Teach both reducers to prune on compact_applied, kept mirrored so cold == live:

1. iOS reducer: on compact_applied, filter the transcript to preservedUuids and append a compact divider item.
2. foldEventDelta: apply the same prune + divider — still purely in-memory, still no re-read.

Both must land together to preserve the cold==live invariant. This pairs with the surviving-UUID metadata work in #13 (own getSessionMessages that preserves compactMetadata / subtype / isCompactSummary off the raw JSONL) — the prune needs that metadata to know which UUIDs survived.

Pointers

• packages/daemon/src/messages/fold.ts — the default: branch comment already documents this deferral.
• packages/app/src/lib/transcript-collection-factory.ts — onEvent compact handling.
• Related: V0.5+: own getSessionMessages — re-implement to preserve compactMetadata, subtype, isCompactSummary #13 (reader preserving compact metadata).