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>

src/starkernel/kernel_main.c

@@ -325,10 +325,10 @@ static void print_banner(void) {
 /**
  * @brief Main kernel entry point after UEFI handoff — executes milestones M0–M6.
  *
- * On amd64, @c kernel_entry.S switches the stack from UEFI's default to a 2 MiB
- * zero-initialised BSS stack and tail-calls this function as @c kernel_main_impl.
- * On aarch64 and riscv64 the assembly trampoline is not yet implemented and the
- * UEFI loader calls @c kernel_main directly.
+ * On amd64 and riscv64, @c kernel_entry.S switches the stack from UEFI's default
+ * to a 2 MiB zero-initialised BSS stack and tail-calls this function as
+ * @c kernel_main_impl. On aarch64 the assembly trampoline is not yet implemented
+ * and the UEFI loader calls @c kernel_main directly.
  *
  * Milestone sequence:
  * - **M0 — Architecture early init** (@c arch_early_init()): On amd64, installs a
@@ -357,7 +357,7 @@ static void print_banner(void) {
  *                   framebuffer descriptor, runtime services pointer, and parsed
  *                   kernel command-line arguments.
  */
-#if defined(__x86_64__)
+#if defined(__x86_64__) || defined(__riscv)
 void kernel_main_impl(BootInfo *boot_info) {
 #else
 void kernel_main(BootInfo *boot_info) {