native_rpi5_entry.S: Pi 5 native (non-UEFI) boot entry stub (FABRIC-3.md §IV.3 item 1)

rpi5_native_start masks x0 down to the documented 32-bit DTB-pointer range
(the firmware's own entry protocol leaves the upper 32 bits unspecified),
stores it into g_rpi5_dtb_ptr for the still-open DTB->BootInfo constructor
(item 2) to read, then switches sp to a dedicated 2 MiB BSS stack -- this
path has no EDK2 boot stack to inherit, unlike every other entry path in
this codebase.

Intentionally halts (wfe/b loop) afterward rather than tail-calling into
item 2's constructor, which doesn't exist yet -- no stub function pretending
to be more than it is.

Not yet linked at the real 0x80000 load address; that needs its own linker
script/build target, not scoped into this item. Compiles and links into the
existing ARCH=aarch64 QEMU/UEFI acceptance build as dead code (ELF kernel
build's KERNEL_ASM wildcards every *.S in arch/aarch64/; nothing there
branches to it), same as rpi5_dtb.c/rpi5_mailbox.c before it.

Verified 3-arch boot to ok>/zuse)ok>.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019YcT3H2PQeyujrzjqS3Var

include/starkernel/rpi5_native_entry.h

@@ -0,0 +1,32 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+  Copyright (c) 2023–2025 Robert A. James. All rights reserved.
+  Licensed under the StarForth License, Version 1.0.
+ */
+
+/**
+ * rpi5_native_entry.h - Raspberry Pi 5 native (non-UEFI) boot entry point
+ * (FABRIC-3.md §IV.3 item 1).
+ *
+ * `native_rpi5_entry.S`'s `rpi5_native_start` is the very first code that
+ * runs on this path -- entered directly by Pi 5 firmware, no UEFI, no
+ * ACPI, none of `boot/uefi_loader.c`'s PE-loader shape applies. It masks
+ * the firmware's raw entry register down to the documented 32-bit DTB
+ * pointer (§IV.1: upper 32 bits of the 64-bit register are unspecified)
+ * and stores it here, then establishes its own dedicated stack (this path
+ * has no EDK2 boot stack to inherit) and halts.
+ *
+ * `g_rpi5_dtb_ptr` is this stub's one real output -- the still-open
+ * DTB->BootInfo constructor (§IV.3 item 2) reads it from here once it
+ * exists; nothing calls that constructor yet, so `rpi5_native_start`
+ * halts rather than tail-calling into a function that isn't real.
+ */
+
+#ifndef STARKERNEL_RPI5_NATIVE_ENTRY_H
+#define STARKERNEL_RPI5_NATIVE_ENTRY_H
+
+#include <stdint.h>
+
+extern uint64_t g_rpi5_dtb_ptr;
+
+#endif /* STARKERNEL_RPI5_NATIVE_ENTRY_H */