net/transport: fix yamux dial fd-reuse race (H1) and outgoing sub-stream leak (H2)

[requilence]

Audit of the net transport stack (net/transport/{yamux,quic}, net/peer, net/pool, net/peerservice, net/secureservice + handshake, net/connutil), cross-checked against hashicorp/yamux 0.1.2, quic-go 0.60.0, libp2p tls 0.48.0, storj.io/drpc, and app/ocache.

Fixes in this PR

H1 — yamux dial closed the raw socket fd (fd-reuse / double-close)

Dial installed a net.Dialer.Control that, on ctx cancellation, ran unix.Shutdown(fd); unix.Close(fd) (and a Windows Closesocket variant) on a descriptor the Go netpoller owns. DialContext(ctx) already aborts the connect and closes its own fd, so the two raced to close the same fd number — the classic fd-reuse hazard, where a concurrently-opened fd can be torn down. Removed the Control/abortOnCancel machinery and the abort_{unix,windows,other}.go files; Dial now relies on net.Dialer.Timeout + DialContext(ctx), which handle timeout and cancellation safely. TestDialContextCancellation still aborts in ~100ms.

H2 — outgoing drpc sub-stream leaked on proto-handshake errors

peer.openDrpcConn opened a yamux/quic sub-stream then ran OutgoingProtoHandshake, returning the error without closing the stream. The handshake closes the conn on I/O errors and ctx cancellation, but not on three protocol-level paths (ErrRemoteIncompatibleProto, non-null ack, ErrUnexpectedPayload), so a version-skewed/misbehaving peer caused unbounded stream accumulation. openDrpcConn now closes the sub-stream on any handshake error, matching the incoming side's defer conn.Close().

Verification

• go build ./net/... and go vet ./net/transport/yamux/... ./net/peer/... — clean
• go test ./net/transport/yamux/... ./net/peer/... ./net/pool/... ./net/peerservice/... — pass
• go test -race -run 'TestDialContextCancellation|TestYamuxTransport_Dial' ./net/transport/yamux/... — pass, no races

Refs GO-7313

[github-actions]

New Coverage	58.0% of statements
Patch Coverage	80.0% of changed statements (8/10)

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