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/arch/riscv64/kernel_entry.S

@@ -0,0 +1,69 @@
+/*
+ * kernel_entry.S — kernel stack setup and entry trampoline (riscv64)
+ *
+ * UEFI hands control to kernel_main() running on EDK2's own boot-time stack.
+ * Unlike amd64 (see arch/amd64/kernel_entry.S, which has carried this same
+ * trampoline since early on), riscv64 previously called straight into
+ * kernel_main_impl with no stack switch at all. The VM bootstrap's call
+ * depth (27 chained word-registration modules, physics/SSM init, capsule
+ * birth, Tripod fleet spawn) overflowed EDK2's riscv64 boot stack partway
+ * through vm_init_with_host()'s return, corrupting the return address and
+ * producing a wild jump shortly after — observed as a load/instruction
+ * page fault (scause 0xd or 0xc) whose exact address drifted between
+ * builds while the crash always landed in this same call region. amd64
+ * never had this problem because kernel_entry.S already switches to a
+ * dedicated 2 MiB BSS stack before anything deep runs; aarch64 apparently
+ * gets away with EDK2's default boot stack being large enough, but that's
+ * incidental, not a guarantee.
+ *
+ * kernel_main is the symbol the UEFI loader calls (extern in uefi_loader.c).
+ * kernel_main_impl is defined in kernel_main.c and contains all the C code.
+ *
+ * Calling convention: RISC-V LP64D — boot_info pointer arrives in a0 and is
+ * passed unchanged to kernel_main_impl via the tail-call.
+ */
+
+#include "starkernel/boot_info_offsets.h"
+
+    /* 2 MiB kernel stack in BSS — fallback when no dynamic stack requested */
+    .section .bss
+    .align 4
+    .global g_kernel_stack
+g_kernel_stack:
+    .space 0x200000
+g_kernel_stack_top:
+    .global g_kernel_stack_top
+
+    .section .text
+    .extern kernel_main_impl
+
+    /*
+     * kernel_main — stack-switch trampoline, then tail-call kernel_main_impl.
+     *
+     * a0 = BootInfo*  (RISC-V LP64D ABI — first argument register)
+     *
+     * Stack selection (checked in order):
+     *   1. boot_info->kernel_stack_base != 0  →  use loader-allocated stack
+     *      (top = base + size, aligned down to 16)
+     *   2. Otherwise  →  fall back to g_kernel_stack (2 MiB BSS)
+     *
+     * Never returns; kernel_main_impl runs the REPL forever or panics.
+     */
+    .global kernel_main
+kernel_main:
+    ld   t0, BOOT_INFO_KERNEL_STACK_BASE_OFFSET(a0)
+    bnez t0, .Ldynamic_stack
+
+    /* BSS fallback */
+    la   t0, g_kernel_stack_top
+    j    .Lstack_ready
+
+.Ldynamic_stack:
+    ld   t1, BOOT_INFO_KERNEL_STACK_SIZE_OFFSET(a0)
+    add  t0, t0, t1          /* top = base + size (stack grows down) */
+
+.Lstack_ready:
+    andi t0, t0, -16         /* 16-byte alignment (RISC-V psABI requirement) */
+    mv   sp, t0
+    mv   s0, zero            /* terminate frame-pointer chain */
+    tail kernel_main_impl    /* a0 (boot_info) already in place */