Remap dead Pfam accessions to their #=GF FW successor instead of dropping their DDIs

[PelzKo]

Summary

Dead Pfam accessions are currently dropped. They could be remapped to their
successor family instead, recovering the DDIs that reference them.

FETCH_DOMAIN_META already reports them correctly — dropped_families.tsv carries
reason dead, and PRUNE_UNREPRESENTED_DDIS removes the DDIs that touch them. So
nothing is wrong today; this is recoverable data being discarded.

The data

Pfam-A.dead.gz (26 KB) carries a forward pointer for every killed family —
measured on the current release: 1121 dead families, 1121 with #=GF FW.

#=GF ID   NtCtMGAM_N
#=GF AC   PF16863
#=GF KL   This family has been killed
#=GF FW   PF13802
#=GF CC   Merged into existing family that has been expanded to cover
#=GF CC   whole structural domain

So PF16863 -> PF13802 is a lookup, not a heuristic.

Dead accessions enter through the DDI sources (3did, PPIDM, Negatome), which are
built against older Pfam releases than the one FETCH_DOMAIN_META streams.

Where the remap has to happen

Downstream, on the DDI rows in the master DB, before EXPORT_UNION_FAMILIES.

Remapping only the instance fetch does not work: the ddi rows would still be keyed
on PF16863 while the instances come back keyed on PF13802, so
PRUNE_UNREPRESENTED_DDIS still deletes them. The rewrite must cover ddi.pfam_a,
ddi.pfam_b and domain.pfam_id.

Implementation notes

• New param url_pfam_dead (https://ftp.ebi.ac.uk/pub/databases/Pfam/current_release/Pfam-A.dead.gz).
• New module, between COLLECT_DDI_DATA and EXPORT_UNION_FAMILIES.
• Collision handling is mandatory: PF16863-X and PF13802-X collapse into one
  DDI once remapped. Needs dedupe on (pfam_a, pfam_b, source, negative), and the
  merge must not silently turn a positive and a negative DDI into one row.
• Forward pointers can chain (A -> B -> C); resolve transitively with a cycle guard.
• Report how many DDIs were remapped and how many collapsed, in
  report_ddi_attrition.py — a remap that quietly halves a source is worse than a drop.
• Semantics caveat worth stating in the docs: #=GF CC usually says "merged into an
  existing family that has been expanded", so the successor family is broader than
  the one the source DDI was annotated against.