Validation Framework: progressive rollout to API repositories and v1 GA

[hdamker]

Problem description

The Validation Framework v1 (#448) is implemented and in RC phase. The
remaining work is to roll it out across all CAMARA API repositories,
promote it from v1-rc to v1 (GA), and retire v0 (pr_validation).

Possible evolution

• All CAMARA API repositories run the v1 caller workflow
• The central default in validation/config/validation-config.yaml
  reaches the stage where validation within the release process is the
  binding gate (per-repo overrides remain available as exceptions, but
  the model is "flip the default for everyone at one point in time")
• v1-rc is promoted to v1 (GA), caller workflow templates are updated
  from @v1-rc to @v1
• v0 (pr_validation) is removed from API repositories and either
  retired or frozen as a fallback
• Codeowners receive PR-time validation results as advisory information;
  the expected behaviour is that only PRs which pass the validation are
  merged into main

Dependencies / relationship to other work

• Commonalities r4.2 release — required for the bundled-spec
  validation path to run cleanly under the production stage
• Validation Framework: regression testing infrastructure #483 (regression testing infrastructure) — provides the safety net
  for moving v1-rc and ultimately v1 without silently breaking rule
  behaviour
• Implement CAMARA Validation Framework v1 #448 (Validation Framework v1 implementation) — the capability
  being rolled out

Additional context

Progress on individual rollout steps will be reported as comments on
this issue.