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)))
         })
 }
 

src/core/plan/java/tests.rs

@@ -69,7 +69,7 @@ fn move_between_packages_rewrites_package_and_all_importers() {
         rewrites
             .iter()
             .map(|r| (
-                r.file.display().to_string(),
+                crate::core::rel_str(&r.file),
                 r.old_text.clone(),
                 r.new_text.clone()
             ))

src/core/plan/mod.rs

@@ -15,7 +15,7 @@ use std::ops::Range;
 use std::path::{Path, PathBuf};
 
 use crate::core::index::Index;
-use crate::core::{JmoveError, JmoveResult, normalize_rel_path};
+use crate::core::{Edit, JmoveError, JmoveResult, normalize_rel_path, rel_str};
 use crate::parser::SourceLanguage;
 
 /// One in-file replacement of an import specifier. Only the specifier text
@@ -32,6 +32,16 @@ pub struct Rewrite {
     pub new_text: String,
 }
 
+impl From<&Rewrite> for Edit {
+    fn from(rewrite: &Rewrite) -> Self {
+        Edit {
+            span: rewrite.span.clone(),
+            old_text: rewrite.old_text.clone(),
+            new_text: rewrite.new_text.clone(),
+        }
+    }
+}
+
 /// Complete plan for moving `source` to `target`.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct MovePlan {
@@ -60,7 +70,7 @@ pub fn plan_move(index: &Index, source: &Path, target: &Path) -> JmoveResult<Mov
     };
     let (source, target) = (rel("source path", source)?, rel("target path", target)?);
     if !index.files.contains(&source) {
-        let s = source.display();
+        let s = rel_str(&source);
         return Err(JmoveError::InvalidArgument(format!(
             "source '{s}' is not an indexed file"
         )));
@@ -71,7 +81,7 @@ pub fn plan_move(index: &Index, source: &Path, target: &Path) -> JmoveResult<Mov
         ));
     }
     if index.files.contains(&target) {
-        let t = target.display();
+        let t = rel_str(&target);
         return Err(JmoveError::PlanRejected(format!(
             "target '{t}' already exists"
         )));
@@ -141,7 +151,7 @@ mod tests {
         let keys: Vec<(String, usize)> = plan
             .rewrites
             .iter()
-            .map(|r| (r.file.display().to_string(), r.span.start))
+            .map(|r| (crate::core::rel_str(&r.file), r.span.start))
             .collect();
         assert_eq!(
             keys,