feat(amneziawg): add embedded amneziawg-go device package (Phase 1)

New internal/amneziawgnet package: builds a real amneziawg-go Device over a
gVisor netstack from an existing amneziawg.Instance, with a TCP/UDP
forwarder that recovers each tunnel connection's real destination and a
peer-identity index keyed by AllowedIPs. This is the foundation for
migrating AmneziaWG off the kernel-module+TPROXY path (see the AmneziaWG-go
vs kernel-module decision) -- nothing wires into live traffic yet, that's
Phase 2 (relay into Xray's own SOCKS5 inbound).

Covered by three real end-to-end tests: a genuine handshake + TCP forwarder
+ identity resolution, the same for UDP (including a reply routed back
through the tunnel), and the manager's reconfigure-in-place vs. rebuild
lifecycle.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

go.sum

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+github.com/amnezia-vpn/amneziawg-go/v3 v3.0.3 h1:XYR85mN53hj2DTzToHs3OxIHrNA59QMg1m3+oiOnBi4=
+github.com/amnezia-vpn/amneziawg-go/v3 v3.0.3/go.mod h1:YoPc6qcOZqD7TXZ1xpedD8Sx3aSKsxN05ZqEFmXDNHk=
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 github.com/andybalholm/brotli v1.2.2/go.mod h1:rzTDkvFWvIrjDXZHkuS16NPggd91W3kUSvPlQ1pLaKY=
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