Fix fire vase hull scalar mapping and restore legible custom rendering

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Motivation

• The custom fire-hull renderer regressed: the vase appeared vertically squished and the hull coloring collapsed to a mostly uniform purple, indicating scalar-to-geometry mapping and display-normalization problems.
• The change aims to repair the rendering pipeline (data -> scalar -> normalization -> mapping -> interpolation -> shading -> projection) for the fire vase while preserving the improved custom interaction model and keeping fixes narrowly scoped to the fire viewer path.

Description

• Map climate scalars to mesh vertices by mesh layer / vertex packing order (repeat per time-ring) in plot_climate_filled_hull to ensure per-vertex alignment with verts_km instead of indexing via raw t_days_vert. (See src/cubedynamics/fire_time_hull.py.)
• Add robust display-only normalization using percentile clipping (default 2nd–98th percentiles with finite fallbacks) and handle NaNs so outliers do not collapse the colormap range. (Renderer-side and fire_plot defaults in src/cubedynamics/verbs/fire.py.)
• Enable interpolation and correct color attachment by setting intensitymode="vertex" and keeping flatshading=False, and add a developer diagnostic scalar_debug_mode="z" to color by z (time) for vertical-banding verification. (See src/cubedynamics/fire_time_hull.py.)
• Restore legibility via a display-only z_exaggeration parameter applied to the Plotly scene aspectratio so the vase reads taller without mutating scientific coordinates, and reduce scene scaffolding (background/grid/zeroline suppressed) for a cleaner presentation. (Exposed through fire_plot and forwarded to the hull renderer.)
• Add a focused unit test file tests/test_fire_hull_viewer_scalars.py to verify scalar-to-vertex ordering, the z diagnostic mapping, and that z_exaggeration is applied.

Testing

• Added tests in tests/test_fire_hull_viewer_scalars.py which validate layer-ordered scalar attachment, scalar_debug_mode="z", and z_exaggeration application.
• Ran the targeted test command pytest -q tests/test_fire_hull_viewer_scalars.py tests/test_fire_plot_cube_first.py and all tests passed in this environment.
• The PR changes are limited to src/cubedynamics/fire_time_hull.py, src/cubedynamics/verbs/fire.py, and the new test file tests/test_fire_hull_viewer_scalars.py.

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Codex Task