Constrain Seurat Pseudobulk feature-ID contract

[maggiecam]

Summary

• constrain feature_id_column to the implemented downstream-safe value
  GeneName in the canonical schema
• reject an invalid CLI value before any Seurat object conversion begins
• clarify the fixed handoff contract in function help and the README
• link the deployment adapter that is currently under review

Compatibility and versioning

This corrects a schema/implementation mismatch identified during independent
review of NIDAP-Community/OMIX-Seurat-Pseudobulk#1. The implementation already
rejected every value other than GeneName, so this is a compatible patch from
module 0.4.0 to 0.4.1 and interface version 1 remains unchanged. Aggregation,
matrix values, manifests, output names, and downstream routing are unchanged.

Supports #68.

Validation

• Rscript modules/OMIX-Seurat-Pseudobulk/tests/test-module-layout.R
• Rscript modules/OMIX-Seurat-Pseudobulk/tests/test-seurat-pseudobulk.R
• Rscript tests/test-monorepo-layout.R
• git diff --check

The broader Seurat-to-DEG integration test was invoked but skipped locally
because OmixSeurat is not installed in the current host library; published
runtime validation remains a separate CI/deployment gate.