[python] Add temporal alignment for multimodal scans

[XiaoHongbo-Hope]

Purpose

Multimodal training data is often stored as independently sampled streams. A
caller currently has to collect those streams and implement timestamp matching,
episode isolation, snapshot pinning, and payload lookup itself.

This draft proposes a typed, bounded temporal-alignment API for
MultimodalTable.scan():

steps = align(
    actions.scan().select(["episode_id", "event_time", "action"]),
    on="event_time",
    by="episode_id",
    sources={
        "camera": nearest(images.scan().select("image"), tolerance=20),
        "state": backward(states.scan().select("state"), tolerance=50),
    },
)

for batch in steps.to_arrow_batch_reader(batch_size=128):
    train(batch)

Source names are caller-defined output namespaces, not schema or domain names.
Using an explicit mapping supports dynamically generated source lists without
coupling this API to a UI specification.

The implementation:

• requires an explicit group boundary so matches never cross episodes/clips;
• supports exact, backward, forward, and nearest matching with inclusive
  tolerance and deterministic earlier-frame tie breaking;
• pins every input scan to its current snapshot;
• plans only group keys, timestamps, and _ROW_ID, then fetches selected
  payload rows in batches;
• keeps BLOB values as descriptors so existing image/video readers can fetch or
  decode only selected samples;
• emits validity, matched timestamp, and signed delta columns for auditing.

This is deliberately a bounded MVP. It does not add interpolation, training
windows, watermark/streaming semantics, a customer-specific JSON spec, or a
generic ScanQuery batch-reader API. The metadata index is currently
coordinator-local. Since this is a draft, feedback on the API shape and whether
the alignment executor belongs in PyPaimon is especially welcome.

Tests

python -m pytest \
  pypaimon/tests/multimodal_temporal_test.py \
  pypaimon/tests/multimodal_table_test.py -q

81 passed

python -m flake8 \
  pypaimon/multimodal/temporal.py \
  pypaimon/tests/multimodal_temporal_test.py