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>

docs/ROADMAP.md

@@ -298,8 +298,13 @@ Design/decision records and a running log of shipped work (items marked
   reuses it verbatim. Periodic (not exact sunrise/sunset timers) for
   robustness to suspend/DST; the idempotent `auto` keeps ticks cheap. Enable
   rebuilds once (installs the timer + `auto --force`); disable is instant
-  (the flag flips and `auto` self-gates). Live timer→switch is a real-session
-  (V3) check.
+  (the flag flips and `auto` self-gates). **V2:** `checks.auto-theme` moves
+  the VM clock across a configured sunset/sunrise and asserts the theme flips
+  day→night→day with the right rebuild count (idempotent between) — the
+  `home-manager switch` itself is stubbed via `NOMARCHY_REBUILD` (that
+  generic apply path is exercised by every manual theme change). The only
+  on-hardware residual is the live user timer firing on its `OnCalendar`
+  schedule (the schedule is `systemd-analyze`-validated).
 - **Nicer shell out of the box:** ✓ zsh is the default login shell, with a
   starship prompt themed from the JSON, autosuggestions + syntax
   highlighting, and modern-CLI ergonomics — `cat`→bat (theme "ansi", so it