fix: handle dotted names and .lua extension in meta path + name check

Two bugs:

1. Meta stem fallback used raw instance name (unslugged), so names with
   forbidden chars like '/' would create bogus directory components in
   the meta path. Fix: fallback now slugifies + init-prefixes, matching
   name_for_inst.

2. AdjacentMetadata name check used split('.').next() to extract the
   filesystem stem, breaking dotted names like "Name.new" (stem became
   "Name", mismatched the instance name, wrote an unnecessary name
   property). Fix: check the conditions that cause name_for_inst to
   diverge (invalid chars or init-prefix) directly instead of comparing
   path stems.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/snapshot_middleware/lua.rs

@@ -166,7 +166,19 @@ pub fn syncback_lua<'sync>(
                         .and_then(|s| s.strip_suffix(ext.as_str()))
                         .map(str::to_owned)
                 })
-                .unwrap_or_else(|| snapshot.new_inst().name.clone());
+                .unwrap_or_else(|| {
+                    let name = &snapshot.new_inst().name;
+                    let base = if crate::syncback::validate_file_name(name).is_err() {
+                        crate::syncback::slugify_name(name)
+                    } else {
+                        name.clone()
+                    };
+                    if base.to_lowercase() == "init" {
+                        format!("_{base}")
+                    } else {
+                        base
+                    }
+                });
             fs_snapshot.add_file(
                 parent_location.join(format!("{meta_stem}.meta.json")),
                 serde_json::to_vec_pretty(&meta).context("cannot serialize metadata")?,