feat(tui): Phase 4 completion + Phase 5 session/lock/text-width

Phase 4 (bd-1df9) — all 5 acceptance criteria met:
- Sync screen with delta ledger (bd-2x2h, bd-y095)
- Doctor screen with health checks (bd-2iqk)
- Stats screen with document counts (bd-2iqk)
- CLI integration: lore tui subcommand (bd-26lp)
- CLI integration: lore sync --tui flag (bd-3l56)

Phase 5 (bd-3h00) — session persistence + instance lock + text width:
- text_width.rs: Unicode-aware measurement, truncation, padding (16 tests)
- instance_lock.rs: Advisory PID lock with stale recovery (6 tests)
- session.rs: Atomic write + CRC32 checksum + quarantine (9 tests)

Closes: bd-26lp, bd-3h00, bd-3l56, bd-1df9, bd-y095

Cargo.toml

@@ -49,6 +49,7 @@ httpdate = "1"
 uuid = { version = "1", features = ["v4"] }
 regex = "1"
 strsim = "0.11"
+which = "7"
 
 [target.'cfg(unix)'.dependencies]
 libc = "0.2"