Gallery: single-week fixture makes monthly/annual plots degenerate

[micahpw]

docs/gat_examples/02_dispatch_monthly_system.py renders a single bar — the fixture covers one week, so a monthly aggregation plot can't demonstrate anything. The example even acknowledges it ('the fixture covers one week').

Options, in order of preference:

1. Extend the fixture horizon so monthly (and annual) views are meaningful — gated on the local annual-solve benchmark currently running (a full 365-step UC+ED solve on the DCP fixture). If an annual solve is cheap enough for CI, use it; otherwise a 2-3 month fixture may be the sweet spot.
2. Failing that, swap the gallery example to an aggregation the one-week fixture can actually demonstrate.

Note: any SIM_STEPS/generate.jl change rotates the fixture cache key — bundle with a deliberate refresh + baseline regen, and iterate locally first (gat-sienna:v4 image is cached locally for exactly this).

[micahpw]

Annual-solve benchmark result (local Apple-Silicon dev machine, Docker, DCP + slacks fixture config, SIM_STEPS=365):

• Wall clock: 281m47s (~4.7 h), clean solve, zero NO_SOLUTION retries across the year
• ≈46 s per daily step (UC + ED + Emulator)

Implication for this issue: extending the RTS fixture to a full year is not CI-viable — GitHub runners are slower than the benchmark machine and the 6 h job limit leaves no margin. A 2–3 month RTS fixture (~45–70 min locally, more in CI) is conceivable as a refresh-workflow-only artifact but heavy. The better path for meaningful monthly/annual gallery views is the 5-bus annual fixture (#18), which should be orders of magnitude faster per step at 5 buses vs RTS's 73.

Recommendation: keep RTS at one week (nodal detail role), pursue #18 for the time dimension, and revisit this issue's gallery example once #18 lands.