client-core: port TranscriptSource and joinPages page-boundary healing

Closes docs/RUST.md's "client-core prerequisites for P1" box: the
cache-vs-server stitching TranscriptSource.kt does, and the
joinPages/healSplitMessage/adoptRun page-boundary healing
TranscriptItems.kt does, both ported into client-core with no UI
framework dependency.

Neither Kotlin file had a JVM unit test of its own, so the port used the
Kotlin source and AGENTS.md's "things that have bitten" paging incidents
as the spec instead of a test-for-test transcription. Both regressions
get a dedicated test: TranscriptSource::page refuses before == 0 before
touching the cache or the network (loadOlderPage's incident), and
adopt_run now runs on every page join rather than only the one where a
split call was found (the "one run drawn as two" incident).

fetch_transcript_lines (api.rs, additive) pairs each transcript line with
the exact server bytes via serde_json::value::RawValue rather than
re-serializing a parsed Value, so a cached line and a live SSE frame for
the same event agree byte-for-byte -- the fetch_transcript_page other
callers under iris/ depend on is untouched.

client-core: 85 -> 109 tests. cargo test/clippy --all-targets/fmt clean.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

client-core/src/lib.rs

@@ -11,5 +11,6 @@ pub mod notifications;
 pub mod sse;
 pub mod transcript_cache;
 pub mod transcript_fold;
+pub mod transcript_source;
 
 pub use event_model::*;