client-core: hoist transcript_fold::{fold_page,raw_seq} out of desktop-app

Both desktop-app's app.rs and the new Android transcript client (RUST.md's
I5) need the same page-fold and live-stream resume-cursor logic; per
CODE_RULES's "write the logic once" it now lives in client-core alongside
fold_event/group_tool_runs instead of being duplicated. desktop-app calls
the shared functions; its own copies and their tests moved with them.

Also fixes a clippy::collapsible_if in config.rs's percent_decode, found
while re-running clippy after this change (let-chains are stable now).

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

iris/desktop-app/src/app.rs

@@ -45,7 +45,9 @@
 
 use client_core::api::{ApiClient, SessionSummary, UreqTransport};
 use client_core::event_stream::{StreamItem, follow_session_events};
-use client_core::transcript_fold::{TranscriptItem, fold_event, group_tool_runs};
+use client_core::transcript_fold::{
+    TranscriptItem, fold_event, fold_page, group_tool_runs, raw_seq,
+};
 use event_model::SeqEvent;
 use iris::prelude::*;
 use std::sync::Arc;
@@ -421,118 +423,3 @@ fn placeholder(rsc: &mut DefaultRsc<Client>, message: &str) -> StrongWidget {
         .add_strong(rsc)
         .any()
 }
-
-/// Folds a page of raw transcript lines (`ApiClient::fetch_transcript_page`'s
-/// `Vec<Value>`) into the flat item list `client_core::transcript_fold`
-/// works over. A line this build can't parse fails the whole page rather
-/// than being skipped -- CODE_RULES's "an enumeration must be able to say
-/// 'it broke'" -- since silently dropping one event could hide, say, the
-/// user message the composer is about to look like it never sent.
-fn fold_page(values: &[serde_json::Value]) -> Result<Vec<TranscriptItem>, String> {
-    let mut items = Vec::new();
-    for value in values {
-        let event: SeqEvent = serde_json::from_value(value.clone()).map_err(|e| {
-            format!("the server sent a transcript line this build couldn't parse: {e}")
-        })?;
-        items = fold_event(&items, &event);
-    }
-    Ok(items)
-}
-
-/// The wire `seq` a raw transcript line carries -- see `select_session`'s
-/// comment on why the live-stream cursor has to be this, not a folded
-/// item's `seq()`.
-fn raw_seq(value: &serde_json::Value) -> Option<u64> {
-    value.get("seq")?.as_u64()
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    fn line(seq: u64, json: serde_json::Value) -> serde_json::Value {
-        let mut obj = json;
-        obj["seq"] = serde_json::json!(seq);
-        obj["ts"] = serde_json::json!(1.0);
-        obj
-    }
-
-    /// The regression for the bug a real `run-headless.sh` screenshot
-    /// found (see `select_session`'s comment): resuming the live stream
-    /// from the last *item's* seq re-delivers the deltas already folded
-    /// into a still-open assistant message, doubling its tail. `raw_seq`
-    /// of the last wire line must be the true high-water mark instead,
-    /// which for a run of deltas is higher than every item's own `seq()`.
-    #[test]
-    fn the_resume_cursor_is_the_last_wire_seq_not_the_last_items_seq() {
-        let values = vec![
-            line(1, serde_json::json!({"type": "userMessage", "text": "hi"})),
-            line(
-                2,
-                serde_json::json!({"type": "assistantText", "delta": "a"}),
-            ),
-            line(
-                3,
-                serde_json::json!({"type": "assistantText", "delta": "b"}),
-            ),
-            line(
-                4,
-                serde_json::json!({"type": "assistantText", "delta": "c"}),
-            ),
-        ];
-        let after = raw_seq(values.last().unwrap()).unwrap();
-        assert_eq!(after, 4);
-
-        let items = fold_page(&values).unwrap();
-        // The folded item keeps the *first* delta's seq (2), which is
-        // exactly the value that must not be used as the resume cursor.
-        let assistant_seq = items
-            .iter()
-            .find(|i| matches!(i, TranscriptItem::AssistantMsg { .. }))
-            .unwrap()
-            .seq();
-        assert_eq!(assistant_seq, 2);
-        assert_ne!(
-            after, assistant_seq,
-            "the fixed bug: these must differ here"
-        );
-    }
-
-    #[test]
-    fn a_page_folds_into_one_settled_assistant_message() {
-        let values = vec![
-            line(1, serde_json::json!({"type": "userMessage", "text": "hi"})),
-            line(
-                2,
-                serde_json::json!({"type": "assistantText", "delta": "hel"}),
-            ),
-            line(
-                3,
-                serde_json::json!({"type": "assistantText", "delta": "lo"}),
-            ),
-        ];
-        let items = fold_page(&values).unwrap();
-        assert_eq!(
-            items,
-            vec![
-                TranscriptItem::UserMsg {
-                    seq: 1,
-                    text: "hi".to_string(),
-                    attachments: Vec::new(),
-                },
-                TranscriptItem::AssistantMsg {
-                    seq: 2,
-                    text: "hello".to_string(),
-                    settled: false,
-                },
-            ]
-        );
-    }
-
-    #[test]
-    fn an_unparseable_line_fails_the_whole_page() {
-        let values = vec![serde_json::json!({"seq": 1, "ts": 1.0, "type": "not-a-real-type"})];
-        let err = fold_page(&values).unwrap_err();
-        assert!(err.contains("couldn't parse"));
-    }
-}