Clean up plotting.spatial_overview

[ecomodeller]

src/modelskill/plotting/_spatial_overview.py has accumulated debt over time. Worth a focused cleanup pass.

Symptoms

• Five TODOs scattered across ~110 lines:
  • L71 — support Gridded ModelResults
  • L78 — better support for multiple models
  • L87 — this is not supported for all model types
  • L98 — group by lonlat bin or sample randomly (track downsampling)
  • L109 — adjust xlim to accomodate text (annotation overflow)
• Explicit raise ValueError for PointModelResult, TrackModelResult, VerticalModelResult — a reject-list rather than an accept-list
• Type hint claims DfsuModelResult | GeometryFM2D | Iterable[...] but duck-typing in the body accepts more
• Duck-typed adapter for GeometryFM3D added in fix: convert 3D geometry to 2D in spatial_overview #646 as a stop-gap — fine for now, but symptomatic
• Mixed control flow (g = m.data.geometry / g = m) with a stray indented comment between branches

Suggested direction

• Make the accept list explicit: which model/geometry types does this function plot, and what does it do for each?
• Decide on GridModelResult support (or document the gap) — the TODO has been there for a while
• Consider whether 3D-aware behavior belongs here or in a model.outline helper on the model classes themselves (each type knows how to draw its own footprint)
• Type hints should match the accepted set, not the duck-typed reality

Out of scope for the fix PR

This issue is the place to design the cleanup; the immediate notebook-test breakage is fixed in #646.