adding script to create a dummy proband for the ravenscroft-rpl project

[SamBryen]

Purpose

Synthesize a 'dummy' male proband gVCF from a pair of parental gVCFs (RD-1168).

The ravenscroft-rpl cohort contains only unaffected parental duos (no probands), so to enable
trio analysis in seqr we create a synthetic proband that inherits, at worst case, every variant
its parents carry:

autosomes / chrX PAR : maternal allele = ALT iff mother carries any ALT
                       paternal allele = ALT iff father carries any ALT
                       e.g. 0/1 + 0/1 -> 1/1 ; 0/0 + 0/1 -> 0/1 ; 0/0 + 0/0 -> 0/0
non-PAR chrX (male)  : hemizygous, maternal-only -> haploid 1 iff mother carries ALT
chrY                 : copy the father's records verbatim (already hemizygous)
chrM                 : copy the mother's records verbatim (maternal inheritance)

The output is a spec-valid GATK-style gVCF with <NON_REF> reference blocks, so that once it is
combined with the real samples it reads as confident hom-ref (0/0) - not missing - at sites that
are variant only in other families. The dummy's reference blocks are the intersection of the two
parents' hom-ref bands, split at the union of their variant sites.

This phase is gVCF synthesis only. Metamist registration (participant/sample/SG creation,
pedigree import, gvcf analysis linking) and the VDS/seqr workflow wiring are handled separately.

Proposed Changes

• NEW src/cpg_seqr_loader/scripts/synthesize_dummy_probands.py
• EDIT pyproject.toml (add pysam), Dockerfile (install pysam)

Checklist

• Version Bump!
• Related GitHub Issue created
• Tests covering new change
• Linting checks pass