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/plan/java/mod.rs

@@ -14,7 +14,7 @@ use std::path::{Path, PathBuf};
 
 use crate::core::index::Index;
 use crate::core::plan::Rewrite;
-use crate::core::{JmoveError, JmoveResult};
+use crate::core::{JmoveError, JmoveResult, rel_str};
 
 /// Rewrite set for moving `source.java` to `target.java` (both
 /// project-relative, validated by the caller).
@@ -28,7 +28,7 @@ pub(super) fn java_rewrites(
     if target.extension().is_none_or(|e| e != "java") {
         return Err(rejected(format!(
             "'{}' is a .java file, the target must keep the .java extension",
-            source.display()
+            rel_str(source)
         )));
     }
     let sdir = source.parent().unwrap_or(Path::new(""));
@@ -40,7 +40,7 @@ pub(super) fn java_rewrites(
         } else {
             Err(rejected(format!(
                 "'{}' has no `package` declaration (default package); it can only be renamed inside its directory",
-                source.display()
+                rel_str(source)
             )))
         };
     };
@@ -50,15 +50,15 @@ pub(super) fn java_rewrites(
     if !sdir.ends_with(Path::new(&pkg_path)) {
         return Err(rejected(format!(
             "package '{pkg}' does not match directory '{}'",
-            sdir.display()
+            rel_str(sdir)
         )));
     }
     let src_root = strip_package_dir(sdir, pkg);
     let rest = tdir.strip_prefix(&src_root).map_err(|_| {
         rejected(format!(
             "target directory '{}' is outside the Java source root '{}'",
-            tdir.display(),
-            src_root.display()
+            rel_str(tdir),
+            rel_str(&src_root)
         ))
     })?;
     let new_pkg = package_of(rest);
@@ -124,7 +124,7 @@ fn file_stem(path: &Path) -> JmoveResult<String> {
         .and_then(|s| s.to_str())
         .map(str::to_owned)
         .ok_or_else(|| {
-            JmoveError::InvalidArgument(format!("invalid file name '{}'", path.display()))
+            JmoveError::InvalidArgument(format!("invalid file name '{}'", rel_str(path)))
         })
 }