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

src/core/index/mod.rs

@@ -45,6 +45,9 @@ pub struct Index {
     pub imports: HashMap<PathBuf, Vec<ResolvedImport>>,
     /// Declared `package` of each Java file that has one.
     pub packages: HashMap<PathBuf, PackageDecl>,
+    /// FQN → file map for every indexed Java class; the fix rules use it
+    /// for candidate lookup (empty when the project has no Java sources).
+    pub java_classes: JavaClassIndex,
 }
 
 impl Index {
@@ -57,6 +60,7 @@ impl Index {
             files: FileSet::default(),
             imports: HashMap::new(),
             packages: HashMap::new(),
+            java_classes: JavaClassIndex::default(),
         };
         index.scan()?;
         // Resolution needs the complete file set (extension/index guessing)
@@ -74,6 +78,7 @@ impl Index {
                 };
             }
         }
+        index.java_classes = java_classes;
         Ok(index)
     }