experience-mine: split oversized sessions at compaction boundaries

Claude Code doesn't create new session files on context compaction —
a single UUID can accumulate 170+ conversations, producing 400MB+
JSONL files that generate 1.3M token prompts.

Split at compaction markers ("This session is being continued..."):
- extract_conversation made pub, split_on_compaction splits messages
- experience_mine takes optional segment index
- daemon watcher parses files, spawns per-segment jobs (.0, .1, .2)
- seg_cache memoizes segment counts across ticks
- per-segment dedup keys; whole-file key when all segments complete
- 150K token guard skips any remaining oversized segments
- char-boundary-safe truncation in enrich.rs and fact_mine.rs

Backwards compatible: unsegmented calls still write content-hash
dedup keys, old whole-file mined keys still recognized.

src/main.rs

@@ -952,7 +952,7 @@ fn cmd_experience_mine(args: &[String]) -> Result<(), String> {
     }
 
     let mut store = store::Store::load()?;
-    let count = enrich::experience_mine(&mut store, &jsonl_path)?;
+    let count = enrich::experience_mine(&mut store, &jsonl_path, None)?;
     println!("Done: {} new entries mined.", count);
     Ok(())
 }