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

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).

Start with docs/gat_examples/02_dispatch_monthly_system.py and inspect how the one-week fixture feeds the monthly and annual plots. Review the local annual-solve benchmark and the SIM_STEPS/generate.jl workflow before choosing a fixture horizon or alternate aggregation. Done means the gallery example demonstrates a non-degenerate view, with deliberate cache refresh and baseline regeneration if the fixture changes.