Update slugify branch audit plan and implement fixes for identified issues

- Marked all todos in the slugify branch audit plan as completed, confirming the successful verification of core invariants and sync processes.
- Introduced a new plan to address critical bugs, correctness issues, and refactors identified during the audit, including enhancements to metadata handling and slugification logic.
- Added tests to ensure proper handling of filenames with forbidden characters and deduplication of instances with colliding names.
- Refactored metadata operations to improve clarity and maintainability, consolidating logic for reading and writing the `name` field in `.meta.json5` files.

src/syncback/meta.rs

@@ -0,0 +1,76 @@
+//! Shared helpers for reading and writing the `name` field in `.meta.json5` files.
+//!
+//! These are used by both `change_processor.rs` (two-way sync renames) and
+//! `web/api.rs` (syncback added/removed instances) to keep meta file handling
+//! DRY and consistent.
+
+use std::fs;
+use std::path::Path;
+
+/// Upsert the `name` field in a `.meta.json5` file.
+///
+/// If the file exists, parses it and merges the `name` key (preserving other
+/// fields like `ignoreUnknownInstances`). If it doesn't exist, creates a new
+/// file with just the `name` key.
+///
+/// Returns `Ok(true)` if the file was written, `Ok(false)` if serialization
+/// failed (logged), or `Err` on I/O failure.
+pub fn upsert_meta_name(meta_path: &Path, real_name: &str) -> anyhow::Result<()> {
+    let mut obj = if meta_path.exists() {
+        match fs::read(meta_path) {
+            Ok(bytes) => match crate::json::from_slice::<serde_json::Value>(&bytes) {
+                Ok(serde_json::Value::Object(map)) => map,
+                _ => serde_json::Map::new(),
+            },
+            Err(_) => serde_json::Map::new(),
+        }
+    } else {
+        serde_json::Map::new()
+    };
+    obj.insert(
+        "name".to_string(),
+        serde_json::Value::String(real_name.to_string()),
+    );
+    let content = crate::json::to_vec_pretty_sorted(&serde_json::Value::Object(obj))?;
+    fs::write(meta_path, &content)?;
+    Ok(())
+}
+
+/// Outcome of attempting to remove the `name` field from a meta file.
+pub enum RemoveNameOutcome {
+    /// The meta file didn't exist or had no `name` field -- nothing changed.
+    NoOp,
+    /// The `name` field was removed; other fields remain. File was rewritten.
+    FieldRemoved,
+    /// The `name` field was the only field. The file was deleted entirely.
+    FileDeleted,
+}
+
+/// Remove the `name` field from a `.meta.json5` file.
+///
+/// If the file becomes an empty object after removal, deletes it entirely.
+/// Returns the outcome so callers can manage filesystem event suppression.
+pub fn remove_meta_name(meta_path: &Path) -> anyhow::Result<RemoveNameOutcome> {
+    if !meta_path.exists() {
+        return Ok(RemoveNameOutcome::NoOp);
+    }
+    let bytes = match fs::read(meta_path) {
+        Ok(b) => b,
+        Err(_) => return Ok(RemoveNameOutcome::NoOp),
+    };
+    let mut obj = match crate::json::from_slice::<serde_json::Value>(&bytes) {
+        Ok(serde_json::Value::Object(map)) => map,
+        _ => return Ok(RemoveNameOutcome::NoOp),
+    };
+    if obj.remove("name").is_none() {
+        return Ok(RemoveNameOutcome::NoOp);
+    }
+    if obj.is_empty() {
+        fs::remove_file(meta_path)?;
+        Ok(RemoveNameOutcome::FileDeleted)
+    } else {
+        let content = crate::json::to_vec_pretty_sorted(&serde_json::Value::Object(obj))?;
+        fs::write(meta_path, &content)?;
+        Ok(RemoveNameOutcome::FieldRemoved)
+    }
+}