docs(dev): plan amendments — runtime port surface, engine-aware doctor, testing plan

[VijitSingh97]

Amends docs/dev/dual-distribution-plan.md with the findings from the runtime-independence review. Dev doc only — no behaviour change.

• Runtime port surface (new section): container-engine independence defined as a short, mapped port list, not an abstraction layer. Names the three engine-neutral contracts (OCI images by digest; the four container-API endpoints behind the socket-proxy allowlists; config.json as the only path to runtime files), the four-item engine-specific residue (firewall implementation, doctor host checks, render target, proxy socket), and the structural guardrails — plus the review rule that a PR growing the surface must update the section.
• Engine-aware doctor (phase 2): doctor's Docker-shaped trio — daemon reachable, docker group, docker.service boot persistence (pithead:720) — misreports under Podman/systemd, and doctor --json is the appliance commit gate. Appliance variants land behind the same check names and --json shape.
• Testing plan (new section): mapped onto the existing four-tier model, no new model. Parity test at tier 1 with derivation mechanics and the closed-set lock (unrecognized compose key = fail); a Podman lane for the tier-3 mini-stack suite; the appliance tier-4 matrix (A/B commit, fault-injection fallback, migration-withhold proof, netavark egress, first-boot, resets, 7-day soak); installer smoke on the release checklist.
• One-line manifest note: image ownership is per-service — the Tari node already pins from a foreign registry.

lint-docs-voice + markdownlint pass.

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