re: diff_optrace.py — engine-vs-VM offset-path divergence oracle

Pure first_divergence + pick_scene_codebase (longest-common-prefix codebase
identification), unit-tested (test_diff_optrace.py, 4/4). CLI loads the engine
jsonl + VM json, isolates the scene's codebase, and reports the first divergence
with the mis-modeled instruction and +/-3 ops of context on each side (opcode
+ rendered line via sys4load).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tools/test_diff_optrace.py

@@ -0,0 +1,28 @@
+# tools/test_diff_optrace.py (plain runner)
+import sys
+from diff_optrace import first_divergence, pick_scene_codebase
+FAILS=[]
+def check(c,m): (FAILS.append(m) or print("FAIL:",m)) if not c else print("ok:",m)
+
+def test_equal_no_divergence():
+    r = first_divergence([0,1,2,3],[0,1,2,3])
+    check(r["index"] is None and r["agreed"]==4, "equal traces -> no divergence")
+
+def test_first_divergence_point():
+    r = first_divergence([0,1,2,9],[0,1,2,3])
+    check(r["index"]==3 and r["a"]==9 and r["b"]==3, "divergence at first differing offset")
+
+def test_prefix_shorter_vm():
+    r = first_divergence([0,1,2,3],[0,1])          # vm ends early
+    check(r["index"]==2 and r["b"] is None and r["agreed"]==2, "shorter VM trace flagged at end")
+
+def test_pick_codebase_by_longest_common_prefix():
+    entries=[{"codebase":100,"offset":0},{"codebase":100,"offset":5},   # cb100: [0,5,...]
+             {"codebase":200,"offset":0},{"codebase":200,"offset":1},{"codebase":200,"offset":2}]
+    check(pick_scene_codebase(entries,[0,1,2])==200, "codebase matching VM prefix chosen")
+
+def main():
+    test_equal_no_divergence(); test_first_divergence_point(); test_prefix_shorter_vm()
+    test_pick_codebase_by_longest_common_prefix()
+    print("FAILURES:",len(FAILS)); return 1 if FAILS else 0
+if __name__=="__main__": sys.exit(main())