Add an 'array' field to the channel-map ch axis

[cboulay]

Summary

Adds an array field to the structured ch axis, so channels can be grouped by physical electrode array rather than by 32-channel connector bank. This is the grouping wanted for per-array rereferencing (LRR/CAR): a bank is a wiring artifact, the array is the implant.

Two channels share an array iff they share a connector — the label prefix before the first -, which the CMP assigns per 64-channel connector (elec1 in elec1-m1-63) — on the same headstage.

("elec1-m1-63", hs=1) -> "hs1-elec1"
("elec1-m1-63", hs=2) -> "hs2-elec1"
("elec1-m1-63", hs=0) -> "elec1"

Why the headstage prefix

The connector label repeats across headstages. Without the prefix, the same .cmp loaded twice (a 256-ch device built from two 128-ch maps) or a bilateral implant would merge two physically distinct arrays into one cluster.

Why the connector, not the whole label stem

The region token (m1, aip) annotates where an array was implanted; a connector whose channels carry inconsistent region labels is still one array. Grouping on the connector also matches the convention already used offline in intent-pipelines (_array_ids_from_metadata), so weights fitted offline cluster the same way when applied live — verified directly on a real 256-ch Hub1 layout (two CMPs, two headstages): both produce an identical partition of 4 arrays x 64 channels.

Fallback

Labels with no connector structure (chan1, or an auto-grid channel with no label) fall back to the connector bank, e.g. ("chan1", hs=1, bank="A") -> "hs1-bankA". Without this, grouping by array would collapse every such channel into a single cluster spanning the whole device. This repo's own test CMP (128ChannelDefaultMapping.cmp) uses chan<N> labels and exercises the fallback.

Populated in three places

• CMP overlays — from entry.label + entry.headstage (+ bank for the fallback)
• Source geometry passthrough — array joins the copied fields when present
• Auto-grid — via the bank fallback, keeping 32-channel clusters

CereLinkSignalSource._build_ch_info also fills it, so a device that read its map from chaninfo carries the field too.

Tests

Five new tests in tests/test_channel_map.py covering identity derivation, CMP population, array-vs-bank group sizes, the two-headstage disambiguation, and the auto-grid fallback. Full suite: 110 passed, 1 skipped.