test(auto-theme): #79 V2 — simulate sunset in a VM (was mis-framed V3)

Bernardo rightly pushed back on deferring #79 to a real session: a
simulated sunset IS VM-testable. Add checks.auto-theme, a runNixOSTest
that installs nomarchy-theme-sync, points NOMARCHY_DEFAULT_THEMES at the
presets, stubs the rebuild via NOMARCHY_REBUILD (a marker), configures
the day/night pair + schedule, then WALKS THE VM CLOCK across the
boundary: 19:55 -> 20:05 -> 20:06 -> next-day 07:30, asserting the active
theme goes summer-day -> night (1 rebuild) -> night (idempotent, still 1)
-> day (2 rebuilds).

This isolates the auto decision + apply + rebuild-trigger; the actual
home-manager switch is stubbed (that generic apply path is exercised by
every manual theme change). The only on-hardware residual is the user
timer firing on its OnCalendar schedule, which systemd-analyze validates.

#79 drops its [blocked:hw] tag — it's V2-verified. ROADMAP updated;
lesson recorded in MEMORY (don't default timer/session features to V3:
stub the generic step, simulate the clock).

Verification: V2 PASS (nix build .#checks.x86_64-linux.auto-theme, fresh,
exit 0); V0 flake check green.

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

flake.nix

@@ -752,6 +752,78 @@
             '';
           };
 
+          # Auto time-of-day theme (#79, VISION § D): simulate crossing
+          # sunset and sunrise by moving the VM clock, and assert
+          # `nomarchy-theme-sync auto` switches the active theme accordingly
+          # — day stays day (no needless rebuild), sunset flips to night,
+          # a later tick is idempotent, sunrise flips back. The actual
+          # `home-manager switch` is stubbed via NOMARCHY_REBUILD (that
+          # generic apply path is exercised by every manual theme change);
+          # this isolates the auto decision + apply + rebuild-trigger.
+          auto-theme = pkgs.testers.runNixOSTest {
+            name = "nomarchy-auto-theme";
+            nodes.machine = { pkgs, ... }: {
+              environment.systemPackages = [ pkgs.nomarchy-theme-sync ];
+              time.timeZone = "UTC";       # naive HH:MM == the clock we set
+            };
+            testScript = ''
+              machine.wait_for_unit("multi-user.target")
+              machine.succeed("timedatectl set-ntp false || true")
+
+              # Writable flake state (start on the day theme) + a rebuild stub
+              # that records each invocation instead of running home-manager.
+              machine.succeed("mkdir -p /root/.nomarchy")
+              machine.succeed(
+                  "echo '{\"slug\":\"summer-day\"}' > /root/.nomarchy/theme-state.json"
+              )
+              machine.succeed(
+                  "printf '#!/bin/sh\\necho switch >> /root/rebuilds\\n' > /root/rebuild"
+                  " && chmod +x /root/rebuild"
+              )
+
+              env = ("NOMARCHY_PATH=/root/.nomarchy "
+                     "NOMARCHY_DEFAULT_THEMES=${./themes} "
+                     "NOMARCHY_REBUILD=/root/rebuild ")
+
+              # Pair + schedule (setup writes only, no rebuild).
+              for kv in ["settings.autoTheme.enable true",
+                         "settings.autoTheme.day summer-day",
+                         "settings.autoTheme.night summer-night",
+                         "settings.autoTheme.sunrise 07:00",
+                         "settings.autoTheme.sunset 20:00"]:
+                  machine.succeed(env + "nomarchy-theme-sync --quiet set " + kv + " --no-switch")
+
+              def slug():
+                  return machine.succeed(env + "nomarchy-theme-sync get slug").strip()
+              def rebuilds():
+                  return int(machine.succeed("wc -l < /root/rebuilds 2>/dev/null || echo 0").strip())
+
+              # Before sunset: already the day theme → no switch, no rebuild.
+              machine.succeed("date -s '2026-01-02 19:55:00'")
+              machine.succeed(env + "nomarchy-theme-sync auto")
+              assert slug() == "summer-day", f"pre-sunset should stay day, got {slug()}"
+              assert rebuilds() == 0, f"no needless rebuild, got {rebuilds()}"
+
+              # Cross sunset → night theme, exactly one rebuild triggered.
+              machine.succeed("date -s '2026-01-02 20:05:00'")
+              machine.succeed(env + "nomarchy-theme-sync auto")
+              assert slug() == "summer-night", f"post-sunset should be night, got {slug()}"
+              assert rebuilds() == 1, f"sunset should trigger one rebuild, got {rebuilds()}"
+
+              # Another tick still after sunset → idempotent, no extra rebuild.
+              machine.succeed("date -s '2026-01-02 20:06:00'")
+              machine.succeed(env + "nomarchy-theme-sync auto")
+              assert slug() == "summer-night"
+              assert rebuilds() == 1, f"idempotent tick must not rebuild, got {rebuilds()}"
+
+              # Cross sunrise the next morning → back to day, second rebuild.
+              machine.succeed("date -s '2026-01-03 07:30:00'")
+              machine.succeed(env + "nomarchy-theme-sync auto")
+              assert slug() == "summer-day", f"post-sunrise should be day, got {slug()}"
+              assert rebuilds() == 2, f"sunrise should trigger a rebuild, got {rebuilds()}"
+            '';
+          };
+
           # Snapshot GUI canary: btrfs-assistant crashes in libbtrfsutil's
           # UNPRIVILEGED subvolume iteration (btrfs-progs 6.17.1, fixed
           # upstream after 6.17.1) but runs fine as root — which is how the