[python][daft] Support timestamp time travel in read_paimon

[kerwin-zk]

Purpose

The Daft connector's read_paimon / explain_paimon_scan previously only supported time travel by snapshot_id and tag_name. This adds a timestamp argument so users can time-travel to the latest snapshot at or before a given time — the third form Paimon already supports at the core level (scan.timestamp-millis / scan.timestamp). Only the Daft wrapper was missing; core is unchanged.

timestamp is dispatched by type:

• int (epoch milliseconds) or datetime.datetime -> scan.timestamp-millis (a naive datetime is interpreted in the local timezone)
• str (e.g. '2026-07-09 10:00:00') -> scan.timestamp

snapshot_id, tag_name and timestamp are mutually exclusive; setting more than one raises ValueError.

read_paimon("db.orders", opts, timestamp="2026-07-09 10:00:00")
read_paimon("db.orders", opts, timestamp=datetime(2026, 7, 9, 10))
read_paimon("db.orders", opts, timestamp=1751990400000)

Docs: docs/docs/pypaimon/daft.md time-travel section updated with the timestamp example and parameter description.

Tests

Added to tests/daft/daft_integration_test.py:

• test_read_paimon_with_timestamp — end-to-end: with a cutoff between two commits, both the millis and datetime forms return only the first snapshot.
• test_timestamp_scan_option_mapping — unit coverage of the type→option mapping (int / str / datetime; bool and float rejected).
• test_read_paimon_rejects_multiple_time_travel — parametrized mutual-exclusion checks (replaces the old two-way check).

All tests/daft/ pass locally (163 passed, 1 skipped); flake8 clean under dev/cfg.ini.