Refactor: extract BaseScan and ManifestGroupPlanner

Pure refactor of the table-scan hierarchy with no behavioural change,
isolating the scan-architecture work from the upcoming incremental
append scan feature (split out of #3364 per reviewer request).

- Introduce BaseScan(ABC) as a superclass of TableScan holding the
  snapshot-independent surface (row filter, options, limit, chaining
  helpers, and the format-converter sinks built on to_arrow). TableScan
  keeps snapshot_id, catalog, table_identifier, snapshot(), use_ref()
  and abstract count(), so its existing surface is unchanged.
- to_pandas/to_polars become concrete defaults on BaseScan and
  to_duckdb/to_ray move up too. This loosens, but does not break,
  TableScan's abstract contract. to_arrow_batch_reader stays concrete on
  DataScan (not abstract on BaseScan) so external TableScan subclasses
  that were valid before still instantiate.
- Extract ManifestGroupPlanner from DataScan and route all of DataScan's
  local planning (scan_plan_helper and _plan_files_local) through it, so
  the partition/metrics/residual evaluators live in exactly one place.
- Preserve all docstrings and explanatory comments.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: smaheshwar-pltr