Fix riscv64 boot crash: add missing kernel stack trampoline

kernel_main on riscv64 ran directly on EDK2's UEFI boot-time stack, with
no dedicated stack switch — amd64 has always had a kernel_entry.S
trampoline for exactly this reason (its own comment: "the FORTH
interpreter + DOE experiment loop can easily exceed that depth").
aarch64 happens to get away without one because its firmware's default
stack is apparently larger, but that was never a guarantee.

On riscv64 the VM bootstrap's call depth (27 word-registration modules
-> physics/SSM init -> Tripod capsule birth) overflowed that small
stack, corrupting a return address and producing a wild jump / page
fault right after vm_init_with_host() returned — reproduced consistently
across the 2026-08-01 DoE campaign logs.

- src/starkernel/arch/riscv64/kernel_entry.S (new): RISC-V stack-switch
  trampoline mirroring amd64's, giving the kernel a dedicated 2 MiB BSS
  stack before anything deep runs.
- kernel_main.c: riscv64 now builds kernel_main_impl (invoked via the
  trampoline) instead of kernel_main directly, same pattern as amd64.
- Makefile.starkernel: wires the new file into the riscv64 build.
- uefi_loader.c: RAW_LOG() was silently a no-op on every non-amd64 arch;
  added a real raw-UART writer for riscv64 (QEMU virt's uart8250 at MMIO
  0x10000000) so existing loader diagnostics actually produce output.

Verified: all three architectures boot clean to [Hera] ok> in the
required order (amd64, aarch64, riscv64); logs and DoE CSVs from these
runs included.

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

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