fix: #149 — eliminate parallel-test flake in runtime::config tests

## Problem

`runtime::config::tests::validates_unknown_top_level_keys_with_line_and_field_name`
intermittently fails during `cargo test --workspace` (witnessed during
#147 and #148 workspace runs) but passes deterministically in isolation.

Example failure from workspace run:
  test result: FAILED. 464 passed; 1 failed

## Root cause

`runtime/src/config.rs::tests::temp_dir()` used nanosecond timestamp
alone for namespace isolation:

  std::env::temp_dir().join(format!("runtime-config-{nanos}"))

Under parallel test execution on fast machines with coarse clock
resolution, two tests start within the same nanosecond bucket and
collide on the same path. One test's `fs::remove_dir_all(root)` then
races another's in-flight `fs::create_dir_all()`.

Other crates already solved this pattern:
- plugins::tests::temp_dir(label) — label-parameterized
- runtime::git_context::tests::temp_dir(label) — label-parameterized

runtime/src/config.rs was missed.

## Fix

Added process id + monotonically-incrementing atomic counter to the
namespace, making every callsite provably unique regardless of clock
resolution or scheduling:

  static COUNTER: AtomicU64 = AtomicU64::new(0);
  let pid = std::process::id();
  let seq = COUNTER.fetch_add(1, Ordering::Relaxed);
  std::env::temp_dir().join(format!("runtime-config-{pid}-{nanos}-{seq}"))

Chose counter+pid over the label-parameterized pattern to avoid
touching all 20 callsites in the same commit (mechanical noise with
no added safety — counter alone is sufficient).

## Verification

Before: one failure per workspace run (config test flake).
After: 5 consecutive `cargo test --workspace` runs — zero config
test failures. Only pre-existing `resume_latest` flake remains
(orthogonal, unrelated to this change).

  for i in 1 2 3 4 5; do cargo test --workspace; done
  # All 5 runs: config tests green. Only resume_latest flake appears.

  cargo test -p runtime
  # 465 passed; 0 failed

## ROADMAP.md

Added Pinpoint #149 documenting the gap, root cause, and fix.

Closes ROADMAP #149.

rust/crates/runtime/src/config.rs

@@ -1254,11 +1254,21 @@ mod tests {
     use std::time::{SystemTime, UNIX_EPOCH};
 
     fn temp_dir() -> std::path::PathBuf {
+        // #149: previously used `runtime-config-{nanos}` which collided
+        // under parallel `cargo test --workspace` when multiple tests
+        // started within the same nanosecond bucket on fast machines.
+        // Add process id + a monotonically-incrementing atomic counter
+        // so every callsite gets a provably-unique directory regardless
+        // of clock resolution or scheduling.
+        use std::sync::atomic::{AtomicU64, Ordering};
+        static COUNTER: AtomicU64 = AtomicU64::new(0);
         let nanos = SystemTime::now()
             .duration_since(UNIX_EPOCH)
             .expect("time should be after epoch")
             .as_nanos();
-        std::env::temp_dir().join(format!("runtime-config-{nanos}"))
+        let pid = std::process::id();
+        let seq = COUNTER.fetch_add(1, Ordering::Relaxed);
+        std::env::temp_dir().join(format!("runtime-config-{pid}-{nanos}-{seq}"))
     }
 
     #[test]