Add interval-based update mode for test dataset generation

Add optional interval-based random content updates to the cdssync
migration test dataset generator and document the new behavior.

This allows the dataset to be created once and then updated either
continuously or every N seconds while preserving the intended
special-case file structure.

cdssync/migration-test-manifest.md

@@ -2,6 +2,18 @@
 
 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.
 
+The generator script can also run in continuous update mode after initial creation. In that mode, mutable content files are rewritten with random data on a fixed interval:
+
+- omit the interval argument to create the dataset once and exit
+- use `0` for continuous rewrites with no sleep between passes
+- use any integer greater than `0` to rewrite mutable files every `N` seconds
+
+Important implementation detail for update mode:
+
+- the update loop rewrites content-bearing regular files that are intended to simulate active data churn
+- it does not rewrite script files, sparse files, symlinks, hard links, or empty files
+- this preserves the special-case filesystem structure while still generating ongoing content changes
+
 ## Recommended Root Layout
 
 - `regular/`