Gallery: capacity overview mangled; load heatmap and LDC empty

Two rendering defects on the docs gallery, likely related to how the sienna v4 fixture data flows into these examples:

1. Generation Capacity Overview (docs/gat_examples/07_capacity_overview.py) renders mangled on the docs site — overlapping/garbled output.
2. Load Heatmap and Load Duration Curve (docs/gat_examples/08_load_heatmap.py) render empty axes — the underlying load query likely returns empty for the fixture rather than raising, so sphinx-gallery happily embeds blank figures.

Repro: build docs against the sienna v4 fixture and inspect gallery pages 07/08. Empty-data cases should either produce data or fail loudly — blank gallery figures are worse than build errors because they ship silently.

Reproduce the docs build against the sienna v4 fixture and inspect docs/gat_examples/07_capacity_overview.py and docs/gat_examples/08_load_heatmap.py. Trace how fixture data reaches each example and verify that the capacity plot renders correctly while load data is present or the empty-data case fails loudly instead of embedding blank figures.