feat: fix engine (Phase 1.6) + git-aware mv + windows-safe path output

- Edit engine: generic span edits (replace/insert/delete) with stale-index
  guard; mv and fix share one atomic apply/rollback/diff/--json pipeline
- jmove mv|check|fix commands; java rules: unused-import, missing-import
  (unique-FQN insert, ambiguous -> candidates for agents), import-order
  (google style; cross-rule byte conflicts prune by severity and converge)
- mv of tracked files goes through git mv (staged rename, --no-git opt-out,
  moved_via/would_move_via in --json, GIT_ERROR code)
- fix java extract_package losing single-segment packages (bare identifier,
  not scoped_identifier) — broke the FQN index for 'package p;' projects
- core::rel_str: project-relative paths rendered with '/' on every platform
  (human/JSON/diff headers/git pathspecs), Path::display() no longer used
  for output; e2e fixtures: java/fix, java/fix_missing, java/order

tests/java/order/src/main/java/com/example/app/App.java

@@ -0,0 +1,12 @@
+package com.example.app;
+
+import java.util.List;
+import com.example.util.Maths;
+import com.example.gone.Unneeded;
+import java.util.Map;
+
+public class App {
+    List<String> a;
+    Map<String, String> b;
+    int v = Maths.compute();
+}

tests/java/order/src/main/java/com/example/app/C.java

@@ -0,0 +1,10 @@
+package com.example.app;
+
+import java.util.Map;
+import java.util.List;
+
+public class C {
+    List<String> a;
+    Map<String, String> b;
+    int v = Maths.compute();
+}

tests/java/order/src/main/java/com/example/util/Maths.java

@@ -0,0 +1,7 @@
+package com.example.util;
+
+public class Maths {
+    public static int compute() {
+        return 42;
+    }
+}