feat: add pre-write immutability filter for private datasets on enclave

[rasswanth-s]

Summary

• Adds an optional pre_write_filter hook to DataSiteWatcherCache that gates every file write/delete in apply_event_message()
• Implements make_private_dataset_immutability_filter() in the enclave package — blocks overwrites/deletes of private dataset files that already exist on disk
• Wires the filter in SyftEnclaveClient.from_config() so all enclave instances enforce immutability automatically

Threat addressed: Once a DO shares private dataset files with the enclave, subsequent re-sends or sync cycles cannot overwrite them — ensuring the data the enclave computes on remains the same as what was originally delivered.

Test plan

• Unit tests for is_private_dataset_path() (positive, public path, wrong subdir, too short)
• Unit tests for the filter (allows first write, blocks overwrite, blocks delete, allows non-dataset paths, allows remaining files in batch)
• Integration test: full quad setup, DO shares dataset, DO re-shares tampered data, enclave retains original
• All 67 existing enclave tests pass
• All 113 syft_bg tests pass