[python][daft] Add Daft-side scan explain diagnostics

[QuakeWang]

Purpose

close: #7998

Daft's Paimon reader already chooses between native Parquet reads and pypaimon fallback internally, but that routing decision was not observable from the public Paimon Daft API. ReadBuilder.explain() only describes the Paimon scan plan, so users could not diagnose whether a slow scan was caused by PK merge, deletion vectors, BLOB columns, non-Parquet format, or pushdown behavior.

This PR adds a structured Daft-side scan explain API:

• explain_paimon_scan(...)
• PaimonTable.explain_scan(...)

The result includes the underlying Paimon scan explain plus Daft reader routing details: native/fallback split and file counts, fallback reasons, pushed/remaining filters, projection/limit pushdown status, and optional per-split reader mode.

The implementation reuses the same scan builder, partition filtering, and native/fallback routing helpers used by PaimonDataSource.get_tasks() to avoid divergence between diagnostics and actual execution.

Tests

• pytest paimon-python/pypaimon/tests/daft/daft_explain_test.py -q
• pytest paimon-python/pypaimon/tests/daft/daft_data_test.py paimon-python/pypaimon/tests/daft/daft_sink_test.py -q

[XiaoHongbo-Hope]

+1

[leaves12138]

LGTM.

I reviewed the Daft scan explain plumbing and the reader-routing diagnostics. The explain path mirrors the real datasource planning path: it builds the same pushdown state, uses the same Paimon scan builder, applies the same partition-filter skipping, and classifies native Parquet vs pypaimon fallback splits with the same routing conditions. The public APIs on read_paimon, PaimonTable, and explain_paimon_scan look consistent.

Validated locally:

• python3 -m py_compile pypaimon/daft/__init__.py pypaimon/daft/daft_catalog.py pypaimon/daft/daft_datasource.py pypaimon/daft/daft_explain.py pypaimon/daft/daft_paimon.py pypaimon/tests/daft/daft_explain_test.py
• python3 -m flake8 --config=./dev/cfg.ini on the changed Daft files

I could not execute the Daft tests locally because this environment does not have daft installed, but the PR's Python checks are green.