feat: 统一 svfs 子命令入口，打包进度回调与对象缓存路径

桌面应用（svfs-studio / svfs-client）要用的前置改动：

* 新增 src/sqlite_vfs/cli/main.py：`svfs <子命令>` 统一入口，旧的独立命令
  （vfspack / vfsunpack / …）保留为别名，两套跑同一段 execute(args)；各子命令
  因此从 sys.exit() 改成返回退出码，便于被复用；
* FolderPacker.pack_folder(progress=…)：每处理一个条目（目录也算）回调一次，
  供 GUI 进度条使用；只有传了回调时才做"只 stat、不读内容"的预扫描，CLI 的
  行为与开销不变；回调抛出的异常一路上抛——GUI 的中断语义就靠它；
* publish.default_cache_dir()：对象缓存目录（客户端设置里可改）；load_manifest
  对"误把服务器根地址当清单填进来"给出可用插件列表的提示，而不是只甩一个
  format=None。

测试：新增 tests/test_cli_main.py、tests/test_pack_progress.py，test_publish.py 补充。

tests/test_pack_progress.py

@@ -0,0 +1,145 @@
+"""打包进度回调（pack_folder 的 progress 参数）。
+
+约定：
+* 每处理一个条目（目录也算）回调一次，``done`` 从 1 递增到 ``total``；
+* ``total`` 是完整遍历一遍得到的条目数，为了拿到它多做一次"只 stat"的预扫描，
+  因此**只有传了回调时**才算，不传回调的路径（CLI）开销一点不变；
+* 回调抛出的异常会一路向上抛——GUI 的中断按钮就靠这个语义。
+"""
+
+import unittest
+
+import helpers
+
+from sqlite_vfs.core import SQLiteVFS
+from sqlite_vfs.folder_packer import FolderPacker
+
+
+class PackProgressTest(helpers.VFSTestCase):
+    def setUp(self):
+        super().setUp()
+        self.src = helpers.make_tree(
+            self.path("src"),
+            {
+                "a.txt": b"aaa",
+                "b.txt": b"bbb",
+                "sub/c.txt": b"ccc",
+                "sub/deep/d.txt": b"ddd",
+                "empty_dir/.keep": b"",
+            },
+        )
+        # 条目数 = 5 个文件（含 empty_dir/.keep）+ 3 个目录（sub / sub/deep / empty_dir）
+        self.expected_entries = 8
+
+    def collect(self, incremental=False):
+        calls = []
+
+        def progress(done, total, rel_path):
+            calls.append((done, total, rel_path))
+
+        vfs = SQLiteVFS(self.path("prog.svfs"), compress=True)
+        try:
+            packer = FolderPacker(vfs, fs_name="prog")
+            with helpers.quiet():
+                ok = packer.pack_folder(self.src, incremental=incremental, progress=progress)
+        finally:
+            vfs.close()
+        return ok, calls
+
+    def test_full_progress_is_one_call_per_entry(self):
+        ok, calls = self.collect()
+        self.assertTrue(ok)
+        self.assertEqual(len(calls), self.expected_entries)
+        self.assertEqual([done for done, _total, _path in calls], list(range(1, len(calls) + 1)))
+
+    def test_total_matches_entry_count(self):
+        _ok, calls = self.collect()
+        totals = {total for _done, total, _path in calls}
+        self.assertEqual(totals, {self.expected_entries})
+
+    def test_progress_paths_are_relative_and_normalized(self):
+        _ok, calls = self.collect()
+        paths = [path.replace("\\", "/") for _done, _total, path in calls]
+        self.assertEqual(sorted(paths), sorted(["a.txt", "b.txt", "sub", "sub/c.txt",
+                                               "sub/deep", "sub/deep/d.txt",
+                                               "empty_dir", "empty_dir/.keep"]))
+
+    def test_incremental_progress_counts_all_entries(self):
+        ok, calls = self.collect(incremental=True)
+        self.assertTrue(ok)
+        self.assertEqual(len(calls), self.expected_entries)
+        self.assertEqual({total for _done, total, _path in calls}, {self.expected_entries})
+
+    def test_result_is_same_with_and_without_progress(self):
+        """带回调不该改变打包结果：条目集合必须一模一样。"""
+        db_with = self.path("with.svfs")
+        db_without = self.path("without.svfs")
+
+        vfs = SQLiteVFS(db_with, compress=True)
+        try:
+            with helpers.quiet():
+                FolderPacker(vfs, fs_name="prog").pack_folder(
+                    self.src, progress=lambda *_args: None
+                )
+        finally:
+            vfs.close()
+
+        helpers.pack(self.src, db_without, compress=True)
+
+        def snapshot(db_path):
+            vfs = SQLiteVFS(db_path, readonly=True)
+            try:
+                return [
+                    (row["path"], row["is_directory"], row["file_size"], row["content_hash"])
+                    for row in vfs.all_entries()
+                ], sorted(row["hash"] for row in vfs.list_blobs())
+            finally:
+                vfs.close()
+
+        self.assertEqual(snapshot(db_with), snapshot(db_without))
+
+    def test_no_precount_when_progress_is_absent(self):
+        """不传回调就不该多做那次预扫描（CLI 的开销必须保持原样）。"""
+        vfs = SQLiteVFS(self.path("nopre.svfs"), compress=True)
+        try:
+            packer = FolderPacker(vfs, fs_name="prog")
+            original = packer._count_entries
+            called = []
+
+            def spy(source_folder):
+                called.append(source_folder)
+                return original(source_folder)
+
+            packer._count_entries = spy
+            with helpers.quiet():
+                self.assertTrue(packer.pack_folder(self.src))
+            self.assertEqual(called, [])
+        finally:
+            vfs.close()
+
+    def test_exception_from_callback_propagates(self):
+        """回调抛异常要能中断打包——GUI 的"中断"就是靠这个语义实现的。"""
+
+        class Stop(Exception):
+            pass
+
+        seen = []
+
+        def progress(done, total, rel_path):
+            seen.append(rel_path)
+            if done >= 2:
+                raise Stop("用户中断")
+
+        vfs = SQLiteVFS(self.path("cancel.svfs"), compress=True)
+        try:
+            packer = FolderPacker(vfs, fs_name="prog")
+            with helpers.quiet():
+                with self.assertRaises(Stop):
+                    packer.pack_folder(self.src, progress=progress)
+            self.assertEqual(len(seen), 2)
+        finally:
+            vfs.close()
+
+
+if __name__ == "__main__":
+    unittest.main()