Add channels -> electrodes -> probes consistency checks

[ree-gupta]

Thanks @effigies and @rwblair for the meeting and feedback yesterday. I have tried drafting a rule for the channels -> electrodes -> probes consistency checks.

What this does

Validates that references from child files resolve in parent files:

• electrode_name in channels.tsv must exist in electrodes.tsv
• probe_name in electrodes.tsv must exist in probes.tsv

Both checks allow n/a (for aux channels without electrodes, or standalone electrodes without probes). We intentionally do not check the reverse direction (parent -> child) since not all electrodes need channels (broken contacts, reference-only, run-specific subsets).

Changes

• context.yaml: Added name column to electrodes association and new probes association with probe_name column
• associations.yaml: Extended selectors so electrodes is resolved when visiting channels.tsv and probes is resolved when visiting electrodes.tsv (follows the coordsystem precedent which already includes electrodes in its selector). Added inherit: true to probes.
• checks/microephys.yaml: Two warning-level child -> parent reference checks

Looking for feedback on

I was not able to get the validator to fire these checks when I broke a dataset by replacing probe_name values in electrodes.tsv with nonexistent names. The schema compiles fine, valid datasets still pass, but the broken dataset did not trigger the new warnings.

Also, the check expression uses length(intersects(...)). When intersects() finds zero overlap it returns false rather than an empty array -- how does length(false) behave? If this is problematic, would a simpler check like the following be preferred?

count(unique(columns.probe_name), "n/a") == length(unique(columns.probe_name))
|| intersects(unique(columns.probe_name), associations.probes.probe_name)

Related #2307