fix(zig): handle empty blobs correctly in crsql_changes (TASK-128, TASK-129)

- Fix changes_vtab.zig to distinguish empty blob (X'') from NULL
- SQLite's sqlite3_column_blob returns NULL for zero-length blobs,
  but col_type is still SQLITE_BLOB - use static empty string as ptr
- Add zig/harness/test-edge-cases.sh: deterministic regression tests
- Integrate edge case tests into test-parity.sh

This fixes the divergence discovered by TASK-127 fuzzing.

Author: subtleGradient