client-core: port the transcript fold (events into rows)

Ports the non-Compose half of app/.../TranscriptItems.kt (TranscriptItem,
foldEvent, runIdFor, settleReply, placePeerNote, splitRun) and
ToolRows.kt (TranscriptRow, groupToolRuns) into transcript_fold.rs, with
7 tests covering delta accumulation, settling, tool-run grouping, a
ToolEnd with no matching start, and a question attaching to its call's
row versus drawing its own.

Not ported: TranscriptUnits.kt's flatten of a row into bounded Compose
list units (a fact about that UI framework, not the transcript), and
joinPages/healSplitMessage/adoptRun (page-boundary healing) -- both
recorded in CLIENT_CORE.md as left for whoever picks this up next.
Also noted there: event_model::Event has no Unknown catch-all, so an
event type this build doesn't recognise fails to parse rather than
degrading to a placeholder row, unlike Events.kt's hand-kept mirror.

cargo test (85 passed), clippy --all-targets and fmt clean.

Co-Authored-By: Claude Sonnet <noreply@anthropic.com>

client-core/src/lib.rs

@@ -8,5 +8,6 @@ pub mod event_stream;
 pub mod highlight;
 pub mod sse;
 pub mod transcript_cache;
+pub mod transcript_fold;
 
 pub use event_model::*;