client-core: port the transcript cache

Ports app/.../TranscriptCache.kt (chunked JSONL directory, suffix/gap
tracking, backwards line reader, damage recovery, eviction) with the
full TranscriptCacheTest suite (18 cases). One correction the port
found in translation: SessionCache::guard's Err branch would have
disabled the whole cache on a single damaged chunk, since a damaged
suffix and a real I/O failure both arrived as Err from the same
closure -- separated so damage discards only the one session, matching
the Kotlin original's separate `catch (e: Damaged)` from
`catch (e: IOException)`.

cargo test (67 passed), clippy --all-targets and fmt clean.

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

client-core/src/lib.rs

@@ -4,5 +4,6 @@
 
 pub mod ansi;
 pub mod highlight;
+pub mod transcript_cache;
 
 pub use event_model::*;