[chore] Scaffold Cargo workspace with three crates

[obchain]

Context

Set up the project's root structure — a Cargo workspace with three crates that all future work plugs into. This is the plumbing every later issue depends on.

Scope

• Root Cargo.toml with [workspace] section and resolver = "2"
• Three member crates: charon-core, charon-scanner, charon-cli
• Shared dependency versions pinned in [workspace.dependencies]
• .gitignore excluding build artifacts, .env, IDE dirs, .DS_Store

Acceptance criteria

• Root Cargo.toml declares [workspace] with three members
• Each crate compiles independently
• cargo build at the workspace root succeeds with no warnings
• target/, .env, and IDE directories are git-ignored

References

• Delivered in commit 4b113e1
• PRD section 3 (Day 1 — Foundation)

[obchain]

Superseded — workspace scaffold shipped in feat/01-scaffold-workspace.