feat(physics): wire spatial index and LP-0 systems

[M1thieu]

Pull Request

Objective

• Align LP’s physics scaffolding with explicit IRL laws/approximations and improve scheduling safety for real-time simulation.
• Clean up docs/licensing and make CI safe on GitHub runners.

Implementation

• Added UnifiedSpatialIndex with correct membership tracking and PreUpdate maintenance.
• Wired Coulomb/thermal systems to the unified index and ordered physics sets.
• Added SI units helpers and smooth force cutoffs; improved stability/diagnostics.
• Updated README + crate READMEs for honest scope and dual-license clarity.
• CI now uses RUSTFLAGS=-Ctarget-cpu=generic to avoid SIGILL on runners.

Testing

• cargo fmt --all
• cargo clippy --workspace -- -A clippy::upper-case-acronyms -A clippy::manual-flatten -A clippy::excessive-precision -A clippy::too-many-arguments
• cargo test --workspace

Visual Changes

• README badges/structure, Simulation Scope, Status/Warning cleanup.
• Added license section and dual-license files.

Technical Notes

• Some systems remain partial (e.g., thermal conduction wiring) and are explicitly documented.
• CI override uses generic CPU flags to prevent runner crashes.
• Related to Electromagnetic System Minimal Implementation #111 (electromagnetism baseline).