docs: update CLAUDE.md, README, and Vietnamese guide for migration step

- CLAUDE.md: document migrator.py algorithm and junction-removal rationale
- README.md: pipeline now 5 steps, add --skip-migrate flag, add migrator.py
  to folder structure, update test count 142 -> 158
- docs/huong-dan-su-dung.md: 5-step pipeline table, new glossary entry,
  updated footer version note

CLAUDE.md

@@ -117,6 +117,20 @@ Pass 2 builds `ok_disk_names` — the set of disk names that already match the s
 
 **`build_server_index` progress callback:** Accepts an optional `progress_fn(current, total, name)` callback. `step_fetch` in `run.py` uses this to print `Indexing N/M: @FolderName` every 25 folders so the log never goes silent during the server scan phase. The library itself never calls `print` — the caller owns the I/O.
 
+### `migrator.py` — mod group migration
+
+Before `step_fetch` runs, `step_migrate` moves locally-downloaded mod folders to match the group assignments in the new `comparison.json`. This avoids re-downloading mods that already exist on disk under a different group when presets are switched (e.g. `A` → `A_v1`).
+
+**Algorithm:**
+
+1. `_build_local_index(downloads)` — scans every `downloads/{group}/@Folder`, reads `meta.cpp` to extract `publishedid`, builds `{steam_id → (group, path)}` and `{norm_name → (group, path)}` maps.
+2. `_build_target_list(comparison)` — flattens `comparison.json` into `[(new_group, steam_id, mod_name)]`.
+3. For each target: locate mod on disk (steam_id first, normalised name fallback); skip if already in correct group or destination exists; remove stale junction from `arma_dir` if present; move folder with `shutil.move`.
+
+**Junction removal is critical:** a stale junction (target moved away) still has the reparse point attribute, so `_is_junction()` returns `True` and `link_group` would skip it as `already_linked` without recreating it at the new path. Removing the junction before the move lets `step_link` recreate it correctly.
+
+**CLI:** `python run.py --skip-migrate` bypasses the step if needed.
+
 ### `update_mods.py` — orphan file removal
 
 After downloading updated files, `update_mods.py` compares every file in the local mod folder against the server's file list and **deletes any local files that no longer exist on the server**. This prevents stale `.pbo` or `.bisign` files from accumulating when a mod's content changes upstream. Each removed file is logged as `[-] orphan removed: <rel_path>` and the final summary line includes an orphan count. The orphan check runs even when no files need downloading (e.g. timestamps match but the local folder has extras).