fix back error message for no older generation (OF-8) — back() now checks for any generation older than current; if none, prints ':: error: already at the oldest generation' and exits cleanly instead of running rollback and surfacing nixos-rebuild's 'build failed'; prev now resolves to newest-older gen, not current-1 — build+tests green

@@ -80,7 +80,21 @@8080 8181     var current: u32 = 0;8282     for (gens) |g| if (g.current) { current = g.number; };83-    const prev = if (current > 1) current - 1 else current;83+84+    // Newest generation older than current. If there is none, we're already at the85+    // oldest — stop cleanly instead of letting rollback fail with 'build failed'.86+    var prev: u32 = 0;87+    var has_prev = false;88+    for (gens) |g| {89+        if (g.number < current and (!has_prev or g.number > prev)) {90+            prev = g.number;91+            has_prev = true;92+        }93+    }94+    if (!has_prev) {95+        output.alreadyOldest();96+        return;97+    }8498 8599     output.backStart(ctx.machine.name, current, prev);86100     if (ctx.cfg.confirm and !output.confirm("")) return;

@@ -223,6 +223,13 @@223223     p("{s}done  gen {s}{d}{s}\n", .{ hdr(), c(BOLD), n, c(RESET) });224224 }225225 226+// `back` has nothing older to roll back to. This is a clean stop, not a build227+// failure — the rollback would otherwise surface nixos-rebuild's confusing228+// 'no profile version older than the current' as a fake build error.229+pub fn alreadyOldest() void {230+    p("{s}already at the oldest generation\n", .{if (color_enabled) ERR_C else ERR_P});231+}232+226233 // Like generationDone but for `back` specifically — a successful rollback earns227234 // the gentle kaomoji. `go` keeps the plain generationDone line.228235 pub fn backDone(n: u32) void {