client-core: an unasked page is not an empty one, and two guarded invariants

Review of 73251d6's port of TranscriptSource/joinPages.

`TranscriptSource::page` answered `before == 0` with an empty `Vec`, which
is the same value it answers "this conversation has no more history" with.
That is the state the Kotlin keeps apart: `loadOlderPage` returns false at
`oldestSeq == 0` *without* touching `moreHistory`, and returns false on an
empty page *by latching it*. Collapsing the two moved AGENTS.md's paging
bug one layer down rather than fixing it. `page` returns `OlderPage` now --
`Events(vec![])` is the start of the conversation, `NothingLoaded` is not
an answer about the conversation at all.

`join_pages`' `debug_assert!` on seq ordering across the boundary is not a
true invariant: a peer note carries the seq its turn began at, which can be
older than the page it arrived in, so an ordinary transcript would have
panicked a debug build there. Replaced with the one the function exists to
enforce -- no tool id surviving in both halves.

`fetch_transcript_lines` stores `RawValue`'s exact server bytes, so the
"neither source can produce a newline" comment in `SessionCache::append`
now rests on the server's serializer staying compact rather than on a
local normalization. Checked with a `debug_assert!` in `append` and
`store_page` rather than trusted.

Tests for the failure half, which the port had none of: a 500 mid-page, a
cached line this build cannot read, and the `after` bound in the case that
actually carries one (the existing test asserted only the case with no
bound). `cargo fmt`, `cargo clippy --all-targets`, `cargo test` (112) clean
in client-core; `cargo check -p desktop-app` clean.

client-core/src/transcript_cache.rs

@@ -361,6 +361,10 @@ impl SessionCache {
             {
                 return Ok(false);
             }
+            debug_assert!(
+                lines.iter().all(|l| !l.contains('\n')),
+                "a stored page's lines must each be one line"
+            );
             fs::create_dir_all(&this.dir)?;
             let kind = if rows { "rows" } else { "raw" };
             let mut content = lines.join("\n");
@@ -389,8 +393,16 @@ impl SessionCache {
                 return Ok(());
             };
             // Written as it arrived. A newline inside it would split one
-            // event into two unreadable halves, but neither source can
-            // produce one.
+            // event into two unreadable halves. No source here can produce
+            // one -- an SSE `data:` field cannot hold a raw newline, and a
+            // fetched line is one element of a compact JSON array -- but
+            // that is a fact about the *server's* serializer rather than
+            // anything this file controls, so it is checked rather than
+            // trusted.
+            debug_assert!(
+                !line.contains('\n'),
+                "a cached transcript line must be one line: {line}"
+            );
             use std::io::Write;
             writer.write_all(line.as_bytes())?;
             writer.write_all(b"\n")?;