hammer-upgrade: upgrades atómicos con generaciones y rollback (Etapa E4, primer corte)

Crate hammer-upgrade + CLI 'hammer upgrade apply|rollback|status'. Aplica un
árbol Stage1/producto del store sobre el FHS vivo dejando una generación
(manifest + backup de bytes previos), atómico fichero-a-fichero con 'current'
como commit-point. Rollback restaura EXACTO al árbol anterior (o pre-upgrades).
Cada cambio al journal (HammerHydrate, replay-able). Verificación opcional del
of_tree esperado (índice de mirror E3 / release firmada). Maneja ficheros +
symlinks. 7 tests unitarios + scripts/upgrade-e2e-test.sh (apply v1->v2->
rollback->rollback contra el binario real). SDD 13 actualizado.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.toml

@@ -7,6 +7,7 @@ members = [
     "crates/hammer-overlay",
     "crates/hammer-journal",
     "crates/hammer-mirror",
+    "crates/hammer-upgrade",
     "crates/hammer-agent",
     "crates/hammer-cli",
     "crates/hammerd",
@@ -27,6 +28,7 @@ hammer-bootstrap = { path = "crates/hammer-bootstrap" }
 hammer-overlay = { path = "crates/hammer-overlay" }
 hammer-journal = { path = "crates/hammer-journal" }
 hammer-mirror = { path = "crates/hammer-mirror" }
+hammer-upgrade = { path = "crates/hammer-upgrade" }
 hammer-agent = { path = "crates/hammer-agent" }
 
 # shared third-party deps, pinned at the workspace level