Add plink_fit_regenie subworkflow

[lyh970817]

Description

Adds the plink_fit_regenie subworkflow to fit REGENIE Step 1 prediction models from PLINK-format genotype bundles.

This subworkflow supports the standard REGENIE Step 1 path and the chunked L0/L1 path, reusing existing REGENIE modules and shared human popgen test data.

Draft dependency: this PR depends on #11768 for regenie/runl1.

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• Remove all TODO statements.
• Broadcast software version numbers to topic: versions - See version_topics
• Follow the naming conventions.
• Follow the parameters requirements.
• Follow the input/output options guidelines.
• Ensure that the test works with either Docker / Singularity. Conda CI tests can be quite flaky:
  • For subworkflows:
    • nf-core subworkflows test plink_fit_regenie --profile docker
    • nf-core subworkflows test plink_fit_regenie --profile singularity
    • nf-core subworkflows test plink_fit_regenie --profile conda