docs: add tsnet migration status — what is left

[GeiserX]

Adds docs/MIGRATION_STATUS.md — a complete "what's left of the tsnet→Rust migration" status:

• Per-subsystem verdicts (PRESENT / PARTIAL / ABSENT) across all 14 major subsystems, each cross-referenced to its open tsr- bead(s).
• Public Device/facade API surface vs Go tsnet.Server/LocalClient — what's implemented, partial, or missing.
• Intentional fork supersets (IPv4-only, fail-closed egress, ring-clean crypto, residential-proxy exit) flagged as design posture, not gaps.
• Downstream daemon engine-asks (ci: bump the github-actions group with 5 updates #1–Responder-side cookie/mac2 issuance (under-load DoS mitigation) #27) status — shipped vs remaining.
• Full open-bead backlog by priority (P0 epic + P2 leverage items + the P3 tail) — with the note that bd list --status open stays the source of truth.

Docs only. Mermaid diagram included; states the conclusions, no sources cited.

Signed-off-by: Sergio sergio@geiser.cloud

Created using Claude Code (Opus 4.8)

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Reviewing files that changed from the base of the PR and between 35e5db2 and da18c83.

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• docs/MIGRATION_STATUS.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/migration-status

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