Add Ailunce HD2 (HR_C7000 / CK803S) board port (miosix v3.01 — proposal)

[fenfir]

⚠️ Version-base note (please read first). This port targets miosix v3.01
(current fedetft/miosix-kernel master). OpenRTX/miosix-kernel is on v2.81,
so this PR's diff includes the large v2.81→v3.01 delta and the port cannot merge
as-is — it depends on v3-era interfaces (interfaces_private/os_timer.h,
IRQsetDefaultConsole, the v3 FastGlobalIrqLock/tickless model) that don't exist
in v2.81. Opened as a proposal / discussion starter for adopting the HD2 v3
port; the actual HD2 work is the 8 commits at the tip (everything under
arch/*/cskyv2, arch/*/hr_c7000, arch/board/hrc7000_hd2, plus small
build/kernel hooks).

Summary

Adds a Miosix port for the Ailunce HD2 handheld digital radio, built on the
HR_C7000 SoC with a C-SKY V2 CK803S CPU — a new CPU architecture for
Miosix. Standard three-layer structure on top of miosix v3.01:

• cpu arch/cpu/cskyv2 — CK803S context switch, IRQ/fault entry, atomics, endianness
• chip arch/chip/hr_c7000 — os-timer, GPIO, delays, cache (no-op), reboot
• board arch/board/hrc7000_hd2 — boot/clock init, BSP, console, linker, settings

Validated on real HD2 hardware: boots, schedules, Thread::sleep, idle CPU sleep,
and console output all confirmed.

What works

Tickless scheduler, threads/sync/timers/delays; a CPU fault handler that decodes
PSR.VEC and dumps EPC/EPSR + registers; stdout/printf/bootlog over UART0; idle
CPU sleep via the C-SKY wait instruction; shutdown()/reboot().

Deliberate, hardware-dictated limitations (see the board README)

• Unikernel only — no processes/userspace/MPU (the HR_C7000 has no MMU).
• No hardware PendSV / software-triggerable IRQ on this SoC: the CK803S core VIC
  is not wired (verified on silicon — a Tspend probe never fired); the external PIC
  delivers all interrupts. A context switch requested under the global lock is
  therefore drained in kernel/lock.h + kernel/thread.cpp — guarded, and a no-op
  on architectures that have PendSV. A generic weak IRQtakeDeferredSwitch() hook
  would be a cleaner upstream mechanism if the maintainers prefer.
• No deep sleep (no deep-sleep-surviving wake source) and no filesystem
  (optional, not wired).
• Hand-rolled os-timer (the DW_apb_timers lack a match register + software-pend IRQ).

Full rationale in miosix/arch/board/hrc7000_hd2/README.md.

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