Build: workspace fails on clean clone due to missing fcctl-core path dependency

[AlexMikhalev]

Problem

Local builds of terraphim-ai fail out-of-the-box even after installing a modern Rust toolchain via rustup.

Current blockers:

1. Missing path dependency
   crates/terraphim_rlm depends on a local path that is not present in a fresh checkout:

# crates/terraphim_rlm/Cargo.toml
fcctl-core = { path = "../../../firecracker-rust/fcctl-core" }

This causes cargo metadata / cargo build to fail with:

• failed to read .../firecracker-rust/fcctl-core/Cargo.toml
• No such file or directory (os error 2)

2. (Previously observed) Workspace uses edition = "2024"
   Older distro-provided Cargo/Rust (e.g. Ubuntu 20.04) may not support 2024 edition unless using rustup/newer toolchain.

Impact

• Contributors cannot compile the workspace or install binaries (e.g. via cargo install --path ...) from a clean clone.
• CI/local dev friction: the build fails before any code changes.

Proposal

Make the repo compile from a clean checkout by one (or more) of:

Option A: Vendor/Fetch firecracker-rust dependency

• Add firecracker-rust as a submodule (or documented clone step) so the expected path exists.

Option B: Remove hard path dependency from default build

• Gate fcctl-core behind a feature flag and disable by default.
• Provide a stub/no-op backend for non-firecracker builds.

Option C: Workspace membership/excludes

• If terraphim_rlm is optional/experimental, consider moving it out of crates/* membership or adding it to exclude until the dependency is resolved.

Acceptance criteria

• cargo metadata succeeds on a fresh clone with rustup toolchain installed.
• cargo build (default workspace members) succeeds without requiring extra sibling repos.
• Optional: document the required toolchain in rust-toolchain.toml.