rust-cargo-fast-iteration-profile

Speed up Rust development iteration when cargo build --release takes 10+ minutes even for one-line edits. Use when: (1) every release rebuild takes minutes despite tiny edits, (2) Cargo.toml has lto = "fat" and/or codegen-units = 1 in [profile.release], (3) you want a production- shaped build for testing without paying the LTO cost on every iteration, (4) you suspect cargo is re-linking the whole binary instead of doing incremental work, (5) developing rfdb/grafema-orchestrator/large Rust binaries where full LTO is enabled for shipped builds. Adds a fast profile that inherits release but disables LTO, raises codegen-units, enables incremental compilation — first build still slow because it populates a fresh target/fast/ dep cache, but every subsequent rebuild drops from ~13 min to ~13 s (≈60×).