Deliver loopback UDP datagrams across processes on the same machine

[brandonpayton]

Contract: POSIX loopback is machine-scoped, not process-scoped

sendto(127.0.0.1:port) from one process must reach a socket bound to that port in another process on the same machine. Kandelo delivered loopback datagrams only within the sender's own process — udp_send_datagram holds a single &mut Process and skipped every endpoint whose pid != proc.pid. So cross-process loopback UDP was silently dropped: unconnected sendto returned success with the datagram lost, and connected sockets got a spurious ECONNREFUSED.

TCP already handled the analogous case (cross_process_loopback_connect, invoked from kernel_connect — this is how nginx→PHP-FPM works over loopback). UDP had no equivalent.

Fix

Add deliver_cross_process_loopback_udp (in syscalls.rs, table-agnostic and unit-testable), invoked from the kernel_sendto / kernel_send / kernel_sendmsg dispatch entry points after the same-process attempt. The dispatch layer owns the global process table, mirroring the existing TCP cross-process pattern; syscalls.rs stays table-agnostic.

• Only engages for AF_INET DGRAM sockets whose effective destination is 127.x. Non-loopback (virtual-network 10.88.x) and same-process delivery are untouched.
• A loopback UDP port binds in exactly one process, and endpoints owned by the sender are skipped, so this never double-delivers a datagram already queued same-process.
• An unconnected sender auto-binds to INADDR_ANY; its source is reported as 127.0.0.1 so the receiver's recvfrom sees a loopback source, matching real semantics.

ABI

No change — no new kernel exports/imports or repr(C) types. scripts/check-abi-version.sh reports the snapshot in sync and ABI_VERSION/snapshot consistent.

Validation

• cargo test -p kandelo --target aarch64-apple-darwin --lib — 964 pass, including new test_udp_loopback_cross_process, which binds a UDP socket in process B, delivers from process A, and reads it back via real sys_socket/sys_bind/sys_recvfrom across two Processes (asserting payload + loopback source addr/port).
• scripts/check-abi-version.sh — snapshot in sync.
• Same-process loopback UDP (test_udp_loopback) and non-loopback paths unchanged (helper is a no-op for them).

Follow-up (not in this PR): a host-level two-process integration test would exercise the guest sendto() syscall end-to-end; the dispatch wiring here mirrors the already-proven TCP cross_process_loopback_connect.

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