Add hopper discharge, rotating drum examples and measurement plane enhancements

[SueHeir]

Summary

• Measurement plane enhancements: Circular plane shape for cylindrical geometries, rolling average statistics for smoothed flow rates, and per-atom-type tracking for multi-material flows
• Cylinder wall angular velocity: Rotating cylinder walls with Coulomb tangential friction — enables rotating drum and mill simulations
• Hopper discharge example: Classic DEM validation case measuring mass flow rate through a slot orifice (Beverloo correlation)
• Rotating drum example: Particles avalanching inside a rotating cylinder at ~10 RPM, demonstrating dynamic angle of repose

Test plan

• cargo test --no-default-features --workspace — all 26 crate tests pass
• cargo clippy --no-default-features -- -D warnings — no warnings
• New unit tests for circular measurement plane bounds and rolling average history
• Run hopper_discharge example and verify mass flow rate output
• Run rotating_drum example and verify avalanching behavior in VTP output
• Compare hopper discharge flow rate against Beverloo correlation prediction

🤖 Generated with Claude Code

[SueHeir]

Maintainer Review: ❌ CHANGES REQUESTED

Good feature work — measurement plane enhancements and cylinder angular velocity are well-designed. However, both example configs have a blocking bug.

Blocking: Invalid TOML config format

Both examples/hopper_discharge/config.toml and examples/rotating_drum/config.toml use:

[comm]
processors = [1, 1, 1]

CommConfig has #[serde(deny_unknown_fields)] and only accepts processors_x, processors_y, processors_z — NOT an array. This will cause a runtime TOML parse error:

unknown field `processors`, expected one of `processors_x`, `processors_y`, `processors_z`

Fix: Replace with:

[comm]
processors_x = 1
processors_y = 1
processors_z = 1

Non-blocking

1. README.md not updated with new examples — please add to examples table
2. Cylinder angular velocity friction uses simplified Coulomb friction (no tangential spring). Fine for rotating drums, but add a code comment noting the simplification
3. Code quality is otherwise excellent — circular plane bounds, rolling average via VecDeque, per-type tracking, 3 unit tests ✓

Please fix the config format and I'll merge.