refactor(server): migrate mod.rs modules to path-style files

crates/server/src/save.rs

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+
+//! Server-side persistence: the durability layer over the `shared` save format.
+//!
+//! `shared::save` owns the pure, in-memory framing (the `SYNR` index and `SYNC` records). This
+//! module owns the filesystem side: reading a `.region` file into memory, mutating its chunks, and
+//! flushing it back to disk crash-safely. The write strategy is a whole-file atomic rewrite
+//! (`.tmp` + fsync + rename); the on-disk format is unchanged, so a later slice can switch to an
+//! append-in-place strategy without a format change.
+
+mod region_file;
+
+pub use region_file::{REGION_SIZE, RegionFile, region_coords, region_path};