feat: scaffold workspace with bench-common, bench-server, bench-runner

- bench-common: KeyExchangeMode enum, BenchRecord NDJSON struct
- bench-server: CLI stub with --mode and --listen
- bench-runner: CLI stub with --mode, --server, --payload-bytes, --iters, --warmup, --concurrency, --out

.gitignore

@@ -10,12 +10,10 @@ target/
 
 # Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
 # More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
-Cargo.lock
+# Cargo.lock
 
 # These are backup files generated by rustfmt
 **/*.rs.bk
 
 # MSVC Windows builds of rustc generate these, which store debugging information
 *.pdb
-
-