Define post-Milestone 6 execution and review gates

[bordumb]

What changed

• Adds the product and go-to-market strategy plus AP-SPEC-027 through
  AP-SPEC-033.
• Adds AP-SPEC-032 for the reproducible Phase 7 release candidate and Phase 8
  exact assurance claim.
• Adds AP-SPEC-033 for independent Phase 9 review, structured findings,
  remediation, independent retest, and the Phase 10 preview gate.
• Reconciles SDK, MCP, custody, design-partner, and commercial-discovery work
  with the tracked Phase 7–15 sequence.
• Adds a Phase 7 owner-decision register with all 11 decisions explicitly
  unresolved.
• Adds a read-only Phase 7 readiness audit mapping the existing release
  workflow, 95-package workspace, subject coverage, provenance, SBOM,
  reproducibility, promotion, and claim gaps to their blocking decisions.

Why

The draft Stage 1–5 product sequence conflicted with the merged post-Milestone-6
plan and could have allowed SDK or consequential-effect work to bypass
release-candidate, assurance-claim, and independent-review gates. Approval
policy and design-partner semantic ownership also needed fail-closed boundaries
before implementation.

Developer impact

This is a documentation and execution-contract PR only. It does not alter
runtime behavior, publish an artifact, create an RC tag, engage reviewers, or
permit a later phase to begin.

Validation

• cargo xtask spec-sync
• Local Markdown link validation across all added documents
• Trailing-whitespace validation
• git diff --cached --check

Affected claims

No shipped capability or public assurance claim changes. The documents narrow
future claim language and require exact artifact, policy, review, and evidence
bindings.

Exclusions

• No release automation or semantic-freeze implementation
• No registry or package publication
• No RC tag creation or promotion
• No independent review engagement
• No SDK, MCP, custody, partner, or commercial product implementation

Remaining gate conditions

Phase 7 remains blocked. All decisions in
docs/plans/PHASE_7_RELEASE_OWNER_DECISIONS.md are unresolved and require
explicit owner approval. Phase 9 requires real independent reviewers. Phase 10
remains blocked until the Phase 9 exit gate permits an explicitly labeled
developer preview.