jarnax: swap udp::Socket for udp::Dispatcher in CyphalUDPInterface (issue-61)

[emrainey]

Progress on #61: replaces the udp::Socket abstraction with udp::Dispatcher in CyphalUDPInterface.

Summary

• CyphalUDPInterface now depends on udp::Dispatcher (Join/Leave/Send) instead of udp::Socket; incoming datagrams are pushed in by the dispatcher implementation (libhypha on target).
• New application-owned nucleo::cyphal::HyphaUdpDispatcher adapts libhypha's UDP callback and multicast TX/RX preparation to udp::Dispatcher. Leave() stops local dispatching because libhypha has no unprepare API.
• GoogleTest suite migrated to MockUDPDispatcher (gmock); still 62/62 in jarnax-cyphal alone.

Verification

• on-host-native-llvm: 21/21 tests pass
• on-host-native-clang: 21/21 tests pass
• on-target-cortex-m4-gcc-arm-none-eabi / on-target-cortex-m7-gcc-arm-none-eabi: build clean
• ./scripts/build-all-presets.py: all working presets pass. on-host-native-gcc fails on Darwin only (Homebrew gcc cannot find its own stdlib headers — documented in AGENTS.md; CI runs that job on Linux where it passes).
• act could not be run locally (no Docker daemon on this host).

Work split

• AI (DeepSeek V4 Flash 0731): migrated tests to MockUDPDispatcher, deleted stale TODO.md, verified all presets, rebased issue-61 onto current develop.
• Prior AI sessions: dispatcher abstraction, interface, hypha adapter, test suite (see issue Encapsulate libudpard in a jarnax::cyphal::Interface implementation #61 comments).
• Human: accepted the Xcode license to unblock local builds.

Remaining per #61

• Refactor CyphalApp onto CyphalUDPInterface (remove inline udpard plumbing)
• Hardware verification via pylink-square-mcp/RTT + yactui
• Final ./scripts/build-all-presets.sh pass before merge