Fix SIDECAR_WITHOUT_DATAFILE for EMG coordsystem files

The previous selector used 'modality != emg' which did not work for
coordsystem files because modality is not reliably set for these files.

This fix excludes coordsystem files when:
- datatype is 'emg' (files in emg/ directory), OR
- the file has a space entity (e.g., space-leftForearm_coordsystem.json)

The second condition allows root-level coordsystem files with space
entities to pass validation. These are used in EMG datasets where
coordinate systems are shared across subjects and sessions.

Example valid patterns:
- /space-leftForearm_coordsystem.json (root level, no sub/ses entities)
- /sub-01/emg/sub-01_space-hand_coordsystem.json (subject level)

Author: neuromechanist

src/schema/rules/errors.yaml

@@ -142,8 +142,9 @@ SidecarWithoutDatafile:
   level: error
   selectors:
     - extension == ".json"
-    - suffix != "coordsystem" || modality != "emg"
+    - suffix != "coordsystem" || !(datatype == "emg" || entities.space)
     # ↑ EMG allows multiple coordsys files per data file (distinguished via `space` entity)
+    # ↑ Also excludes root-level coordsys files with `space` entity (inherited by all sessions)
 
 # BIDS Validator Original Issue Code #97
 MissingSession: