Clarify fire visualization architecture and define fire_plot recovery path

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Motivation

• The repo had drifted into two overlapping visualization paths (v.plot() custom cube viewer vs v.fire_plot() Plotly hull) which caused confusion and fragile future changes.
• The goal is to make the current state honest, prevent ad hoc backend switching, and provide a clear, low-risk recovery path toward a single, convergent viewer model.

Description

• Add a new guidance section to AGENTS.md titled Visualization architecture and fire plot recovery plan that documents the transition state, separation-of-concerns, and agent implementation rules. (AGENTS.md)
• Update the fire recipe docs/recipes/fire_event_vase_hull.md with a new Current viewer status section that verbatim-states that v.fire_plot() returns a fire-event bundle plus a Plotly hull figure and is not yet rendered through the custom v.plot() backend. (docs/recipes/fire_event_vase_hull.md)
• Add a short transition note to the cube viewer docs clarifying that v.plot() is the canonical custom viewer and specialized workflows may temporarily use separate backends. (docs/viz/cube_viewer.md)
• Add a new developer architecture note docs/dev/fire_plot_architecture.md describing current layering, stable separations, migration strategy, anti-patterns, and recommended next incremental PRs. (docs/dev/fire_plot_architecture.md)
• Tidy doc-level text so code/docs agree: clarify the fire_plot docstring in src/cubedynamics/verbs/fire.py and update docs/api/verbs.md to reflect that fire_plot(...) returns a Plotly fig_hull today. (src/cubedynamics/verbs/fire.py, docs/api/verbs.md)
• Add the new dev doc to the MkDocs nav. (mkdocs.yml)

Testing

• Ran targeted unit tests with pytest tests/test_fire_plot_loader_calls.py tests/test_fire_plot_cube_first.py tests/test_plot_verb.py -q; the suite passed (15 passed).
• Built the docs with mkdocs build --strict; the build completed successfully (warnings about nav coverage and an upstream MkDocs warning were emitted but did not fail the build).

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Codex Task