RUST-REF-006: Consolidate Rust NAT and runtime safety internals

[itlezy]

RUST-REF-006 - Consolidate Rust NAT and runtime safety internals

Summary

Remove obsolete NAT backends now that MiniUPnPc is the sole supported provider,
then harden synchronous runtime state and native-code boundaries without
changing eD2K/Kad behavior.

Scope

• Remove the deprecated rupnp backend, its ssdp-client dependency, personal
  Git patches, and the unimplemented upnp_igd stub.
• Keep a bounded configuration migration path that accepts MiniUPnPc and rejects
  retired backend identifiers with a clear error.
• Adopt non-poisoning parking_lot::Mutex for short synchronous runtime state;
  keep asynchronous locks only where a guard must cross .await.
• Fail closed explicitly for security-sensitive state instead of relying on
  mutex poisoning.
• Audit every unsafe boundary, especially Gateway: Send, and enable
  unsafe_op_in_unsafe_fn plus documented-unsafe-block enforcement.
• Narrow broad dead-code and unused-import suppressions after reference and
  feature-matrix proof.

Acceptance Criteria

• MiniUPnPc is the only compiled and configurable UPnP provider.
• rupnp, ssdp-client, and the unimplemented IGD provider are absent from
  source, manifests, and the lockfile.
• Short synchronous runtime state cannot cascade through mutex poisoning.
• Security-sensitive failure paths are explicitly fail-closed.
• Every unsafe block and unsafe implementation has a concrete safety proof.
• Broad lint suppressions are removed or reduced to justified items.

Validation

• NAT provider unit tests plus MiniUPnPc discovery/map/release coverage.
• Supported workspace Rust format, Clippy, build, tests, Kad swarm, and VPN leak
  gates.

[itlezy]

MiniUPnPc-only NAT consolidation landed in emulebb-rust commit 9fa0e9a. Removed the deprecated rupnp backend, ssdp-client and personal Git patches, the unimplemented IGD stub, and obsolete diagnostic switches. Added explicit retired-backend config errors. Validation: Rust policy, format, all-target Clippy, and the locked workspace test gate (excluding isolated local_kad_swarm) all passed.

[itlezy]

Runtime safety hardening landed after the MiniUPnPc consolidation: 6348bb0 (credit ledger), 1c81311 (Kad rate limiter), e334933 (Kad peer state), 8ad2668 (Kad RPC), and b56bc95 (eD2K connection budget) use non-poisoning short-lived locks. e1c3705 removes the unnecessary unsafe Gateway Send implementation. 2c97349 makes all 14 crates deny unsafe operations in unsafe fns and undocumented unsafe blocks, with concrete FFI safety comments. Focused subsystem tests and the repository quick gate pass.