Parallelize the test gate; cache cold-init tests, drop the benchmark from it (#139)

run-tests.janet runs the same file set as `jpm test` across a pool of worker
processes (one `janet FILE` each, ev-based). The full gate goes from ~790s
serial to ~98s here (8x), and more on CI where the heavy files don't thrash on
swap. CI and the docs point at it; `jpm test` still works serially.

Three things dominated the wall:

- Nine integration tests cold-built a compile ctx (~8s each); switch them to
  api/init-cached so they share the prebuilt image. The cache key already
  fingerprints the ctx-shaping env vars, so the direct-link ones share one DL
  image and the rest share the plain one.
- core-bench's main ran on every gate (~35s of benchmark loops that assert
  nothing); gate it behind JOLT_BENCH=1.
- cli-test spawned `janet src/jolt/main.janet` ~20 times at ~8s cold each
  (340s under parallel load, and it was the whole wall); prefer build/jolt
  (~20ms baked ctx) when present, fall back to from-source for an unbuilt tree.

type-check-test stays on cold init: a snapshot-loaded ctx loses the success
checker's op/msg detail (jolt-vley). jolt-pria tracks caching from-source
startup generally, which would let cli-test drop the build/jolt preference.

Co-authored-by: Yogthos <yogthos@gmail.com>

.github/workflows/tests.yml

@@ -54,4 +54,7 @@ jobs:
         run: jpm build
 
       - name: Run tests
-        run: jpm test
+        # Parallel runner (same file set as `jpm test`, across CPU workers).
+        # The executable is built in the previous step, so binary-spawning tests
+        # don't skip.
+        run: janet run-tests.janet