Signed audit checkpoint and external anchoring primitives (evidence-grade roadmap)

Migrated from: https://github.com/jeremi/registry-platform/issues/56
Original author: @jeremi
Source repository: `jeremi/registry-platform`
Source issue: `#56`
Source labels: post-1.0
Source milestone/release intent: _none_
Target area/path: `products/platform; crates/registry-platform-*`

## Issue

[redacted private/internal deployment detail during migration]

Both products already stub toward this: relay posture exposes `audit.checkpoint_status` / `verification_status: "not_supported"` and warns `relay.audit_checkpoint_unavailable`; the hash-chain and keyed-integrity primitives live in `registry-platform-audit`. The shared checkpoint/anchoring primitive belongs here so relay and notary adopt one implementation.

[redacted private/internal deployment detail during migration]

**Acceptance:** checkpoint generation + verification round-trip tests; posture fields flip from `not_supported`; continuity-break detection has a failing test before implementation.

_Note: one or more private/internal references were redacted during migration._

## Migration Metadata

- Migrated to the public monorepo on 2026-06-25.
- Source title, body, labels, milestone/release intent, and pre-migration discussion were preserved where available.
- Code-grounded audit note: Posture fields exist, but signed checkpoints and external anchoring remain unsupported/planned.
- Private/internal references, secrets, and obvious deployment-only details were redacted instead of copied forward.

Start by reading the existing hash-chain and keyed-integrity primitives in `registry-platform-audit`, then inspect `products/platform` and `crates/registry-platform-*` for `audit.checkpoint_status`, `verification_status`, and `relay.audit_checkpoint_unavailable`. Establish a failing continuity-break test before implementation. Done means signed checkpoint generation and verification round-trip tests pass, and the posture fields no longer report `not_supported`.

Written by the indexing model from the issue text.