Fix include/version.h collision between hosted Makefile and Makefile.starkernel

The hosted Makefile writes include/version.h with FORCE as a prerequisite
(always regenerates), but Makefile.starkernel's own rule had no
prerequisite at all -- Make only rebuilds a target with no prerequisites
when the file is missing. Since both Makefiles write the same path with
incompatible content (the hosted version has no LITHOS_VERSION/
LITHOS_VERSION_STR at all), running a bare `make -f Makefile.starkernel`
after a hosted `make` build silently reused the wrong file and failed
deep in kernel_main.c with "LITHOS_VERSION_STR undeclared".

Added FORCE (declared .PHONY, matching the hosted Makefile's own existing
pattern) as include/version.h's prerequisite in Makefile.starkernel.
Verified the fix directly: poisoned version.h with a hosted build, then
ran a bare (non-clean) kernel build and confirmed it self-heals.

Full three-architecture acceptance: amd64 (logs/20260922-215333/),
aarch64 (logs/20260922-215803/), riscv64 (logs/20260922-220217/) -- all
three reach [zuse@Hera] ok>, zero UNKNOWN WORD.

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

Makefile.starkernel

@@ -597,6 +597,7 @@ KERNEL_OBJS := \
 .PHONY: qemu qemu-esp qemu-gdb
 .PHONY: thumbdrive iso-usb
 .PHONY: info help
+.PHONY: FORCE
 
 # ==============================================================================
 # MAIN TARGETS
@@ -627,7 +628,15 @@ clean-kernel:
 # BUILD RULES
 # ==============================================================================
 
-include/version.h:
+FORCE:
+
+# FORCE prerequisite (matches the hosted Makefile's own pattern, Makefile:613/626):
+# this file is also written by the hosted Makefile with different, incompatible
+# content (no LITHOS_VERSION/LITHOS_VERSION_STR) -- without FORCE, a bare build
+# here after a hosted `make` run silently reuses that stale/wrong-flavored file
+# and fails with "LITHOS_VERSION_STR undeclared" deep in kernel_main.c, instead
+# of regenerating its own correct version.
+include/version.h: FORCE
 	@mkdir -p include
 	@BUILD_TS=$$(date -Iseconds 2>/dev/null || echo "unknown"); \
 	printf '#ifndef STARFORTH_VERSION_H\n#define STARFORTH_VERSION_H\n\n' > $@; \