chore: add VS Code Dev Container + pin toolchain

[Mathious6]

Why?

Make onboarding and remote dev consistent: same Rust toolchain, editor settings, and basic extensions for everyone = no local setup surprises.

What?

• Add .devcontainer/devcontainer.json (Rust image, zsh, formatOnSave, rust-analyzer using clippy, TOML formatter alignment, default formatters).
• Auto-install clippy & rustfmt via postCreateCommand.
• Ship minimal extensions: rust-analyzer, vscode-lldb, even-better-toml, fill-labs.dependi.
• Add rust-toolchain.toml pinning stable + components (rustfmt, clippy).
• Normalize Cargo.toml formatting (align keys: no functional changes).

Main impact?

• Faster first-run and uniform tooling across contributors.
• Reproducible formatting/linting out of the box.
• No behavior change to builds or runtime, dev-only improvements.

What next?

• Add CI checks for cargo fmt --check and cargo clippy -D warnings (?)
• Small README section.

[azerpas]

LGTM, thanks for your first PR 🥳