feat(installer): add transactional P0-P9 state machine

docs/guides/upgrade-safety-and-recovery.md

@@ -12,6 +12,20 @@ with no snapshot to fall back to.
 Protection is layered. Each layer is independent; a later layer catches what an
 earlier one misses.
 
+## Layer 0 — Transaction journal (install-wide recovery)
+
+The unified installer opens a private journal under
+`${XDG_STATE_HOME:-~/.local/state}/mosaic/install/` before the first target
+mutation. Every mutation scope records its path, prior snapshot, and reversal
+instructions before it is touched. Journal write/sync failure is fatal, and P9
+seals successful journals with a SHA-256 sidecar. See
+[Installer state machine and recovery](./installer-state-machine.md).
+
+This transaction journal is distinct from the retained operator-only backup
+below. The transaction journal is required for correctness and rollback;
+Layer 2's durable backup remains a separately stated, fail-open recovery bonus
+for a manifest bug that the normal transaction did not detect.
+
 ## Layer 1 — Manifest-owned sync (prevention)
 
 The single source of truth for ownership is