graph, node, store: Add incremental dump support

When a dump directory already contains a previous dump (metadata.json),
subsequent dumps only write new rows (VID-append) and generate clamp
files for mutable tables whose block_range was closed since the last
dump. On restore, clamps are loaded into memory and applied to rows
before DB insertion.

Key changes:
- Add clamp Arrow schema (vid + block_range_end)
- Add #[serde(default)] clamps field to TableInfo for backward compat
- Make dump_entity_table and dump_data_sources incremental via prev param
- Layout::dump detects previous dumps, validates deployment match and
  guards against reorgs
- Add load_clamps() to read clamp parquet files into a vid->end map
- Apply clamps during restore in import_entity_table and import_data_sources
- Add DumpReporter::start_clamps/finish_clamps with CLI progress output