chore: start transition layout — python into version-python/, scaffold version-rust/

- git mv of all existing files into version-python/ (history preserved)
- version-rust/: minimal cargo scaffold (edition 2024), no deps yet
- root AGENTS.md documents the transition rules and artifact/schema contract
- main is untouched; the python layout there stays as-is

version-python/tools/plan_diff.py

@@ -0,0 +1,26 @@
+#!/usr/bin/env python3
+"""Diff old vs new sorter plans by source path -> target."""
+import json
+import sys
+
+changed = 0
+same = 0
+examples = []
+for n in (1, 2, 3):
+    old = {p["source"]: p for p in json.load(open(f"/tmp/plan{n}.json"))}
+    new = {p["source"]: p for p in json.load(open(f"/tmp/plan{n}b.json"))}
+    for src, np in new.items():
+        op = old.get(src)
+        if not op:
+            continue
+        if op["target"] != np["target"] or op["kind"] != np["kind"]:
+            changed += 1
+            if len(examples) < 40:
+                examples.append((n, op, np))
+        else:
+            same += 1
+print(f"same: {same}  changed: {changed}")
+for n, op, np in examples:
+    print(f"\n[{n}] {op['name']}  (score {op['score']} -> {np['score']})")
+    print(f"  old: {op['target']}")
+    print(f"  new: {np['target']}")