fix(bench): reset instance JVM settings for sessions without --jvm

apply_jvm wrote instance.cfg and never restored it, so B5-B12a in run 1
ran on B4's last set (Shenandoah, 6 GB). Within-block verdicts hold
(same JVM on both sides); B7's no-shader baseline is re-run in run 2.

Also: B14 spawn-count automation (b14/: fresh seeded world per run,
offline-located biome sites, datapack entity counts, journaled config
variants) and RESULTS-proxy.md with run-1 verdicts.

analysis/client-perf/harness/tests/test_bench_flags.py

@@ -281,3 +281,14 @@ class ShaderLoaded(unittest.TestCase):
         self.assertEqual(bench.shader_flags("X.zip", ok), [])
         self.assertEqual(bench.shader_flags("X.zip", []), ["shader_not_loaded"])
         self.assertEqual(bench.shader_flags("X.zip", bad), ["shader_not_loaded"])
+
+
+class ResetJvm(unittest.TestCase):
+    def test_reset(self):
+        with tempfile.TemporaryDirectory() as d:
+            d = Path(d)
+            (d / "instance.cfg").write_text("JvmArgs=-XX:+UseShenandoahGC\nMaxMemAlloc=6144\n")
+            self.assertFalse(bench.reset_jvm(d))
+            (d / "instance.cfg.harness-orig").write_text("JvmArgs=\nMaxMemAlloc=8192\n")
+            self.assertTrue(bench.reset_jvm(d))
+            self.assertEqual((d / "instance.cfg").read_text(), "JvmArgs=\nMaxMemAlloc=8192\n")