docs: record v0.1.2 adversarial validation and v0.1.3 hardening candidate

[Fatboy-coder]

Summary

Documents the 2026-09-23 maintainer-run adversarial validation of published PPGP v0.1.2 and updates the roadmap.

Findings recorded

• no reproducible core protocol semantic failure demonstrated in the tested scenarios;
• keep v0.1.2 as the current official release;
• treat v0.1.3 as the next hardening candidate, not an already-released version;
• prioritize parser/diagnostics, repository-root resolution, branch visibility and documentation/regression hardening;
• do not promote richer v0.2 coordination primitives without new reproducible evidence.

Scope

Documentation only. No spec change, package version bump, release, tag, or publication.