Implement Decision Point Groups removal (DPG spec)

[ahouseholder]

Summary

Remove the deprecated ssvc.dp_groups package now that DecisionTable provides a complete replacement. Work is split into three phases: removing the four redundant SSVC role group modules, migrating policy_generator.py to use DecisionTable, and (once CVSS backfill issues land) removing the CVSS group collections. Ships as a major-version bump.

Acceptance Criteria

• AC-1: Delete dp_groups/ssvc/ modules (supplier, deployer, coordinator_triage, coordinator_publication, collections) and base.py (DPG-01)
• AC-2: Remove ssvc.dp_groups from IMPORTABLES in utils/defaults.py and delete src/test/dp_groups/ (DPG-01)
• AC-3: Migrate PolicyGenerator to accept DecisionTable instead of DecisionPointGroup (DPG-02)
• AC-4: Remove DecisionPointGroup from doctools.dump_schemas (DPG-02)
• AC-5: Delete dp_groups/cvss/ once issues Backfill DecisionTable objects for CVSS v1 #922, Backfill DecisionTable objects for CVSSv2 #923, Backfill DecisionTable objects for CVSS v3 #924 are merged (DPG-03)
• AC-6: Retain data/schema/v2/DecisionPointGroup*.schema.json as static files; do not regenerate (DPG-03)

Reference

Spec: specs/dp-groups-removal.yaml
Notes: notes/dp-groups-removal.md

Blocked by (Phase 3): #922, #923, #924