pg::wal::receive: make lsn_to_timeline public

External WAL-processing tools that stream replication (e.g. the wal-listener
synchronous receiver) need to resolve which timeline owns a resume LSN by
walking a `TIMELINE_HISTORY` response — exactly what the private
`lsn_to_timeline` already does. They currently duplicate the parser byte-for-
byte. Expose it (follow-up to #19, "Expose replication + object-storage
primitives") so the logic lives in one place.

No behavior change; the existing unit test still covers it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: achudnovskij

src/pg/wal/receive.rs

@@ -649,8 +649,8 @@ async fn get_start_timeline(
 /// Timeline owning `lsn` per a `.history` file. Each row is
 /// `<tli>\t<switch_lsn>\t<comment>`; a switch to `tli+1` happened at
 /// `switch_lsn`, so the first row whose switch LSN exceeds `lsn` names its
-/// timeline, else the file's own (wal-g LSNToTimeLine)
-fn lsn_to_timeline(content: &[u8], lsn: u64, file_timeline: u32) -> u32 {
+/// timeline, else the file's own (wal-g LSNToTimeLine).
+pub fn lsn_to_timeline(content: &[u8], lsn: u64, file_timeline: u32) -> u32 {
     let text = String::from_utf8_lossy(content);
     let mut rows: Vec<(u32, u64)> = Vec::new();
     for line in text.lines() {