lrr cluster by field

[kylmcgr]

Summary

LRR can now derive its per-bank regression blocks from a structured channel-axis
field instead of relying on positional block_size.

• Adds cluster_by_field to SelfSupervisedRegressionSettings (inherited by
  LRR). When set (e.g. "bank") and no explicit channel_clusters are given,
  clusters are derived from that field via
  ezmsg.sigproc.util.channels.channel_clusters_from_field, cached at
  _reset_state (so the message-less _get_channel_clusters — also called
  externally — can return them).
• Precedence: explicit channel_clusters > cluster_by_field > block_size

  single cluster. Default None preserves current behavior.

• _hash_message folds the field's bytes into the hash so derived clusters
  re-derive when the field changes (mirrors the sigproc fix).

Tests

TestClusterByField in test_ssr.py: bank-derived weights byte-identical to
explicit per-bank channel_clusters, explicit-clusters precedence, fallback to
block_size, and re-derive on bank change. 21 pass.

⚠️ Before merge

• Depends on the sigproc helper — land the ezmsg-sigproc car-bank-aware PR
  first.
• pyproject.toml currently pins ezmsg-sigproc to the car-bank-aware branch
  in [tool.uv.sources] for local testing.