chore: initialize Cargo workspace and finalize crate structure

[miguemagicdev]

Summary

This PR transitions the project into a Cargo Workspace, completes the configuration for the physcontinuum-core crate, and removes legacy artifacts from the initial Rust migration.

Changes

• Workspace Management: Created a root Cargo.toml to manage multiple crates and centralize dependency versions.
• Crate Configuration: * Finalized physcontinuum-core/Cargo.toml to inherit from the workspace.
  • Explicitly mapped the library entry point to src/lib.rs.
  • Added missing license headers for legal/compliance consistency.
• Source Cleanup: * Created the initial src/lib.rs for the core crate.
  • Deleted crates/physcontinuum-core/src/core.cpp (obsolete legacy file).
• Build Artifacts: Generated the initial Cargo.lock to ensure reproducible builds across environments.

Why this is necessary

Moving to a workspace structure allows for better modularity as the project grows and ensures that physcontinuum-core is properly recognized as a library crate by the Rust compiler. Deleting the old C++ source prevents linker confusion and reduces repository bloat.

Verification Results

[x] cargo check passes.
[x] cargo metadata correctly identifies the workspace members.
[x] Workspace-level linting/formatting is now functional.