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>

agent/JOURNAL.md

@@ -17,6 +17,25 @@ Template:
 
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+## 2026-07-10 — #79 V2: checks.auto-theme (simulated sunset) — was mis-framed V3
+- **Task:** Bernardo challenged the "V3 real-session" framing — a simulated
+  sunset IS VM-testable. He's right; I'd conflated "needs a real desktop"
+  with "needs the generic home-manager switch."
+- **Did:** Added `checks.auto-theme` runNixOSTest: installs nomarchy-theme-sync,
+  points `NOMARCHY_DEFAULT_THEMES` at presets, stubs the switch via
+  `NOMARCHY_REBUILD` (a marker), configures the pair+schedule, then **walks
+  the VM clock** 19:55→20:05→20:06→next-day 07:30 asserting slug goes
+  summer-day → night (1 rebuild) → night (idempotent, still 1) → day (2). The
+  self-gating I relied on: presets via env, wallpaper/notify no-op headless,
+  run_switch honors NOMARCHY_REBUILD + waybar nudge is check=False.
+- **Verified:** **V2 PASS** — `nix build .#checks…auto-theme` fresh, exit 0.
+  Dropped #79's `[blocked:hw]`; it's done (V2). Updated ROADMAP.
+- **Lesson:** don't default "timer/session feature" to V3 — stub only the
+  generic step (the real HM switch, covered elsewhere) and simulate the rest
+  (clock) headlessly. Only the literal timer-fires-on-OnCalendar is on-HW,
+  and its schedule is systemd-analyze-validated.
+- **Next suggestion:** NEXT head #76 `[blocked:hw]`; else QA / PROPOSED.
+
 ## 2026-07-10 — #79 slice 3: Look & Feel › Auto theme menu (#79 complete)
 - **Task:** BACKLOG #79 slice 3 — the menu; completes the feature.
 - **Did:** `rofi.nix` — "Auto theme (on/off)" row in Look & Feel + an