Add versioning rules for DecisionTable objects

[ahouseholder]

Currently, SSVC's DecisionTable objects lack versioning rules. This is a significant omission — without versioning conventions, it is unclear how to track changes to decision tables, how to communicate compatibility, or how to deprecate old versions. We should define versioning semantics (e.g., semantic versioning), rules for when a version bump is required, and how versions should be represented in the Python model and JSON/CSV data files.

[ahouseholder]

✅ Ingested.

• Docs PR: specs: ingest idea #1143 — DecisionTable versioning rules #1144 (targets specs_bootstrap / Bootstrap formal specification infrastructure, agent skills, and codebase reference docs #1142)
• Implementation issue: Implement DecisionTable versioning rules (VR-05) #1145

Design decisions captured in specs/versioning.yaml (new VR-05 group, VR-05-001 through VR-05-007). Also: VR-01-006 downgraded SHOULD → MAY; VR-02 tightened to DecisionPointGroup-only.