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/bench.py

@@ -778,6 +778,16 @@ def apply_jvm(jvm, heap_mb):
     cfg.write_text("\n".join(out) + "\n")
 
 
+def reset_jvm(inst_dir=None):
+    """Restore instance.cfg from the pre-harness copy (undoes an earlier --jvm/--heap)."""
+    d = Path(inst_dir) if inst_dir else INST_DIR
+    orig = d / "instance.cfg.harness-orig"
+    if orig.exists():
+        shutil.copyfile(orig, d / "instance.cfg")
+        return True
+    return False
+
+
 def plan_dir():
     return GAME / "saves" / WORLD / "datapacks" / PLAN_PACK
 
@@ -1234,6 +1244,8 @@ def cmd_session(a):
         apply_pins()
         if a.jvm or a.heap:
             apply_jvm(a.jvm, a.heap)
+        else:
+            reset_jvm()  # --jvm/--heap must not leak into later sessions
         apply_shader(GAME, journal, a.shader, a.shader_options)
         apply_dh(GAME, journal, a.dh == "on")
         disable_mods(GAME, journal, to_disable)