feat(volume_mesh): add prism and pyramid cell support (upstream PR #353)

[xarthurx]

Summary

Ports upstream C++ Polyscope PR #353 (commit dcbaedb) — adds prism (6-vertex wedge) and pyramid (5-vertex) cells to `VolumeMesh`, reaching parity with all four upstream cell types.

• New public API: `register_prism_mesh`, `register_pyramid_mesh`, `VolumeMesh::new_prism_mesh`, `VolumeMesh::new_pyramid_mesh`, `slice_prism`, `slice_pyramid`.
• `VolumeMesh::cell_type` switches to sentinel-count classification (0/2/3/4 → Hex/Prism/Pyramid/Tet) matching upstream.
• Cell shape data (face polygons, triangulation stencils, tet-decomposition patterns) extracted to a new `cell_data` submodule so each per-cell loop dispatches through a single `face_data_for(cell_type)` table — collapses ~90 lines of tet-vs-hex branching while making all 4 cell types first-class.
• New example: `volume_mesh_mixed_cells_demo` registers one of each cell type side-by-side.

Changes

Commit	What
`9cbb85f`	Scaffold `cell_data` submodule + enum variants
`a6697d1`	Sentinel-count cell_type detection
`57fb27d`	Tet-decomposition + centroid for prism/pyramid
`9010c0b`	Refactor 7 face-iteration sites to dispatch via `cell_data`
`fd2256f`	`slice_prism` / `slice_pyramid` via tet decomposition
`8937f2b`	Public `register_prism_mesh` / `register_pyramid_mesh`
`7cee690`	Mixed-cell-type demo example
`be474a0`	Update feature-status.md + CHANGELOG
`57a3321`	rustfmt pass
`c454089`	Simplify pass: faster `cell_type`, no per-cell `Vec` in decomposition, slice helper extraction, fix UI label that was miscounting prisms as hex

Test Plan

• `cargo test --workspace` — 214 passed, 2 ignored
• `cargo clippy --workspace -- -D warnings` — zero warnings
• `cargo fmt --check` — clean
• New tests cover: cell-type detection for all 4 types, prism/pyramid tet decomposition, prism/pyramid centroid, single-cell exterior face counts, mixed-cell mesh face counts, prism/pyramid slice geometry, public constructors
• Manual visual check (please run before merging): `cargo run --release --example volume_mesh_mixed_cells_demo` — verify the four shapes render correctly side-by-side, each with its "z" scalar quantity selectable, and picking highlights individual cells

Notes

• File-size budget: `volume_mesh/mod.rs` net change is −90 lines (the dispatch refactor offset all four new cell-type code paths). Stays well under the 2000-line limit.
• Cross-cell decomposition consistency for prism/pyramid uses the same diagonal-split rule as upstream so shared faces in mixed meshes tessellate identically.

🤖 Generated with Claude Code

[xarthurx]

@copilot resolve the merge conflicts in this pull request

[Copilot]

Pull request overview

Ports upstream Polyscope PR #353 to add prism (wedge) and pyramid cell support to VolumeMesh, expanding mixed-cell functionality (cell-type detection, face iteration/rendering, slicing) to cover all four upstream cell types.

Changes:

• Added new prism/pyramid registration + constructors and a mixed-cell demo example.
• Refactored per-cell face/tet decomposition logic into a new cell_data table and switched cell_type detection to sentinel-count classification.
• Extended slice helpers to support prism/pyramid via tet decomposition, with accompanying tests and documentation updates.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
examples/volume_mesh_mixed_cells_demo.rs	New example showing tet/hex/prism/pyramid side-by-side.
docs/feature-status.md	Updates feature parity notes to include prism/pyramid support.
crates/polyscope/src/volume_mesh.rs	Adds register_prism_mesh / register_pyramid_mesh public API.
crates/polyscope/Cargo.toml	Registers the new example target.
crates/polyscope-structures/src/volume_mesh/slice_geometry.rs	Adds slice_prism / slice_pyramid and shared slicing helper.
crates/polyscope-structures/src/volume_mesh/mod.rs	Adds new cell types, constructors, sentinel-based cell_type, and dispatches loops via cell_data.
crates/polyscope-structures/src/volume_mesh/cell_data.rs	New centralized face/triangulation + tet-decomposition tables per cell type.
CHANGELOG.md	Documents the new cell types, APIs, and sentinel-based classification change.

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[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved by merging origin/main into this branch and fixing the conflicts in CHANGELOG.md and docs/feature-status.md so both sets of updates are preserved. Addressed in commit 2f21074.