Add cdssync migration test dataset tooling

Add the cdssync migration test dataset manifest, generator script,
workspace instructions, and gitignore.

This sets the default workflow to:
- generate the dataset locally
- copy it to the test machine with metadata preserved
- verify the copied data before migration testing

cdssync/migration-test-manifest.md

@@ -0,0 +1,127 @@
+# Migration Test Dataset Manifest
+
+This manifest defines a compact, high-value filesystem test set for validating file migration behavior. It is intended to cover common file-content, naming, metadata, and directory edge cases without generating an unnecessarily large corpus.
+
+## Recommended Root Layout
+
+- `regular/`
+- `hidden/`
+- `spaces in name/`
+- `deep/tree/level1/level2/level3/`
+- `readonly-dir/`
+- `links/`
+- `metadata/`
+- `empty-dirs/`
+
+## Test Objects
+
+### Regular Files
+
+- `regular/text_1mb_644.txt`
+- `regular/text_3mb_600.txt`
+- `regular/text_5mb_755.txt`
+- `regular/random_1mb_600.bin`
+- `regular/random_3mb_644.bin`
+- `regular/random_5mb_755.bin`
+- `regular/compressible_1mb_644.log`
+- `regular/compressible_3mb_600.log`
+- `regular/compressible_5mb_755.log`
+- `regular/script_1mb_755.sh`
+- `regular/script_3mb_700.sh`
+- `regular/script_5mb_755.sh`
+- `regular/sparse_1mb_600.img`
+- `regular/sparse_3mb_600.img`
+- `regular/sparse_5mb_600.img`
+- `regular/empty_000_644.txt`
+- `regular/empty_001_600.txt`
+- `regular/empty_002_755.txt`
+
+### Hidden Files
+
+- `hidden/.hidden_text_1mb_644.txt`
+- `hidden/.hidden_random_3mb_600.bin`
+- `hidden/.hidden_script_1mb_755.sh`
+- `hidden/.hidden_empty_644`
+- `hidden/.hidden_sparse_5mb_600.img`
+
+### Files With Spaces
+
+- `spaces in name/file with spaces text 1mb 644.txt`
+- `spaces in name/file with spaces random 3mb 600.bin`
+- `spaces in name/file with spaces script 1mb 755.sh`
+- `spaces in name/file with spaces empty 644`
+- `spaces in name/file with spaces sparse 5mb 600.img`
+
+### Long-Name Files
+
+- `regular/longname_aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa_text_1mb_644.txt`
+- `regular/longname_bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb_random_3mb_600.bin`
+- `regular/longname_cccccccccccccccccccccccccccccccc_compressible_5mb_755.log`
+
+### Deep Path Files
+
+- `deep/tree/level1/level2/level3/deep_text_1mb_644.txt`
+- `deep/tree/level1/level2/level3/deep_random_3mb_600.bin`
+- `deep/tree/level1/level2/level3/deep_script_1mb_755.sh`
+- `deep/tree/level1/level2/level3/deep_sparse_5mb_600.img`
+
+### Duplicate-Content Cases
+
+- `regular/dup_source_text_3mb_644.txt`
+- `regular/dup_copy_a_text_3mb_600.txt`
+- `deep/tree/level1/level2/dup_copy_b_text_3mb_755.txt`
+
+### Timestamp Variants
+
+- `regular/old_text_1mb_644.txt`
+- `regular/recent_text_1mb_644.txt`
+- `regular/futureish_text_1mb_644.txt`
+
+### Read-Only Or Awkward Placement Cases
+
+- `readonly-dir/locked_text_1mb_444.txt`
+- `readonly-dir/locked_random_3mb_400.bin`
+- `readonly-dir/locked_script_1mb_500.sh`
+
+### Links
+
+- `links/symlink_to_text_1mb_644.txt`
+- `links/symlink_to_deep_random_3mb_600.bin`
+- `links/symlink_to_hidden_file`
+- `links/hardlink_to_random_3mb_644.bin`
+- `links/hardlink_to_compressible_5mb_755.log`
+
+### Directories
+
+- `empty-dirs/empty_a/`
+- `empty-dirs/empty_b/`
+- `empty-dirs/.hidden_empty_dir/`
+- `readonly-dir/no_write_subdir/`
+- `deep/tree/level1/level2/level3/`
+
+### Metadata Cases
+
+These should only be created if the source filesystem supports them and the test environment allows them.
+
+- `metadata/xattr_text_1mb_644.txt`
+- `metadata/xattr_random_3mb_600.bin`
+- `metadata/acl_text_1mb_644.txt`
+- `metadata/acl_script_1mb_755.sh`
+
+## Approximate Storage
+
+Estimated real disk usage for this manifest:
+
+- core allocated files: about `95 MiB` to `125 MiB`
+- with filesystem overhead and modest headroom: plan for about `150 MiB`
+- comfortable reserve for later additions: `250 MiB`
+
+Important notes:
+
+- sparse files may report a logical size of `1 MiB` to `5 MiB` while using much less physical disk space
+- symlinks, hard links, directories, ACLs, xattrs, and empty files add little compared with regular allocated files
+- if you later expand this set with more size permutations or more metadata variants, storage will grow mostly with the fully allocated non-sparse files
+
+## Usage Recommendation
+
+Use this directory as the canonical definition of the source dataset. Generate the files once, preserve the original unchanged, and transfer a copy to the source test machine using metadata-preserving tooling such as `rsync -aH`, `cp -a`, or a tar archive workflow.