process_sub: add UUID v4 to FIFO path; fixes test_fifo_path_unique race

The pre-existing FIFO-path generator
  /tmp/vsh-fifo-<pid>-<counter>
collided when two ShellState instances shared a pid (e.g., parallel
cargo tests where each test creates a fresh state with the AtomicUsize
counter starting at 0). The first mkfifo wins; the second hits
AlreadyExists and races the first test's cleanup. Surfaced as an
intermittent failure of `process_sub::tests::test_fifo_path_unique`
during this session's batch-7 verification run; passed in isolation.

Fix: append a UUID v4 suffix:
  /tmp/vsh-fifo-<pid>-<counter>-<uuidv4>

uuid v1.22 with the v4 feature is already a dep of this crate
(originally for operation IDs in audit logs). The pid + counter
remain useful for grep/debugging discrimination; the UUID makes
collisions cryptographically improbable across:
  * two parallel tests in the same binary
  * two `vsh` processes that happen to share a recycled pid
  * two FIFOs in the same process (counter handles this anyway)

Verified locally:
  cargo build  -> EXIT 0
  Stress loop: 10/10 runs of the process_sub::tests:: suite all pass
                (was failing intermittently when called via full
                `cargo test` due to parallel scheduling).
  Full suite: 703 passed / 0 failed / 14 ignored — unchanged.

This is a real production fix, not just a test fix: the same
`/tmp/vsh-fifo-<pid>-<counter>` collision could affect production
when multiple containerised vsh instances share a pid namespace.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

impl/rust-cli/src/process_sub.rs

@@ -40,10 +40,16 @@ impl ProcessSubstitution {
         cmd: String,
         state: &mut ShellState,
     ) -> Result<Self> {
-        // Generate unique FIFO path: /tmp/vsh-fifo-<pid>-<counter>
+        // Generate globally-unique FIFO path:
+        //   /tmp/vsh-fifo-<pid>-<counter>-<uuidv4>
+        // The pid + counter make collisions improbable in single-process use;
+        // the UUID v4 suffix prevents collisions when two ShellState
+        // instances share a pid (e.g., parallel cargo tests where each test
+        // creates a fresh state with counter starting at 0).
         let pid = std::process::id();
         let fifo_id = state.next_fifo_id();
-        let fifo_path = PathBuf::from(format!("/tmp/vsh-fifo-{}-{}", pid, fifo_id));
+        let unique = uuid::Uuid::new_v4().simple();
+        let fifo_path = PathBuf::from(format!("/tmp/vsh-fifo-{}-{}-{}", pid, fifo_id, unique));
 
         // Create FIFO using mkfifo(2) syscall
         #[cfg(unix)]