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/api.rs

@@ -10,6 +10,7 @@
 
 use std::io::Read;
 
+use event_model::SeqEvent;
 use serde::Deserialize;
 use serde_json::Value;
 
@@ -116,6 +117,14 @@ impl<T: Transport> ApiClient<T> {
         Self { transport }
     }
 
+    /// The transport underneath, for a caller that needs the raw SSE
+    /// stream (`event_stream::follow_session_events`) rather than one of
+    /// this client's typed REST calls -- `transcript_source::TranscriptSource`
+    /// is the one that does.
+    pub fn transport(&self) -> &T {
+        &self.transport
+    }
+
     fn json_request<R: for<'de> Deserialize<'de>>(
         &self,
         method: &str,
@@ -266,15 +275,72 @@ impl<T: Transport> ApiClient<T> {
         limit: u32,
         coalesce: bool,
     ) -> Result<Vec<Value>, ApiError> {
-        let mut path = format!("/sessions/{session_id}/transcript?limit={limit}");
-        if let Some(before) = before {
-            path.push_str(&format!("&before={before}"));
-        }
-        if coalesce {
-            path.push_str("&coalesce=true");
-        }
-        self.json_request("GET", &path, None)
+        self.json_request(
+            "GET",
+            &transcript_path(session_id, before, limit, coalesce, None),
+            None,
+        )
     }
+
+    /// A page of transcript history, each line handed back paired with the
+    /// exact text it came from, and bounded below by `after` -- the shape
+    /// `crate::transcript_source::TranscriptSource` needs to store what it
+    /// fetched in the transcript cache without a second round trip to fetch
+    /// the raw text separately. Ported from `Api.kt`'s `fetchTranscript`.
+    ///
+    /// Uses [`serde_json::value::RawValue`] rather than re-serializing a
+    /// parsed [`Value`], so the stored line is the exact bytes the server
+    /// sent (key order and float literal included) rather than this
+    /// crate's own idea of how to write them back out -- the cache and a
+    /// live SSE frame must agree byte-for-byte on the same event, which is
+    /// exactly what caught the `serde_json` float-rounding bug this
+    /// project's `AGENTS.md` records.
+    pub fn fetch_transcript_lines(
+        &self,
+        session_id: &str,
+        before: Option<u64>,
+        limit: u32,
+        coalesce: bool,
+        after: Option<u64>,
+    ) -> Result<Vec<(String, SeqEvent)>, ApiError> {
+        let path = transcript_path(session_id, before, limit, coalesce, after);
+        let raw: Vec<Box<serde_json::value::RawValue>> = self.json_request("GET", &path, None)?;
+        raw.into_iter()
+            .map(|value| {
+                let line = value.get().to_string();
+                let event: SeqEvent = serde_json::from_str(&line).map_err(|e| ApiError {
+                    message: format!(
+                        "the server sent a transcript line this build couldn't parse: {e}"
+                    ),
+                    status: None,
+                })?;
+                Ok((line, event))
+            })
+            .collect()
+    }
+}
+
+/// The query string shared by [`ApiClient::fetch_transcript_page`] and
+/// [`ApiClient::fetch_transcript_lines`], so the two agree on how each
+/// parameter is written rather than keeping two copies to drift.
+fn transcript_path(
+    session_id: &str,
+    before: Option<u64>,
+    limit: u32,
+    coalesce: bool,
+    after: Option<u64>,
+) -> String {
+    let mut path = format!("/sessions/{session_id}/transcript?limit={limit}");
+    if let Some(before) = before {
+        path.push_str(&format!("&before={before}"));
+    }
+    if coalesce {
+        path.push_str("&coalesce=true");
+    }
+    if let Some(after) = after {
+        path.push_str(&format!("&after={after}"));
+    }
+    path
 }
 
 /// The blocking [`Transport`] backed by `ureq`, the same crate `server/`