feat: add build numbering

kernel/GNUmakefile

@@ -225,6 +225,18 @@ override OBJ += $(addprefix obj-$(ARCH)/,$(NASMFILES:.asm=.asm.o))
 endif
 override HEADER_DEPS := $(addprefix obj-$(ARCH)/,$(CFILES:.c=.c.d) $(CXXFILES:.cpp=.cpp.d) $(OBJCFILES:.mm=.mm.d) $(ASFILES:.S=.S.d))
 
+# Build number. src/Api/BuildNo.hpp holds MONTAUK_BUILD_NUMBER and is included
+# (transitively) only by Api/Syscall.cpp. We regenerate (bump) it whenever any
+# kernel object OR any userspace source changes, so the number embedded in the
+# kernel tracks real rebuilds of the kernel/userspace. Api/Syscall.cpp.o is
+# excluded from the prerequisites to avoid a dependency cycle (it depends on the
+# header), and a clean build bumps exactly once.
+override BUILDNO_HDR := src/Api/BuildNo.hpp
+override BUILDNO_OBJ := obj-$(ARCH)/Api/Syscall.cpp.o
+override PROG_SRC := $(shell find ../programs/src ../programs/include ../programs/lib \
+    -type f \( -name '*.c' -o -name '*.cpp' -o -name '*.h' -o -name '*.hpp' \
+    -o -name '*.S' -o -name '*.asm' \) 2>/dev/null)
+
 # Default target. This must come first, before header dependencies.
 .PHONY: all
 all: bin-$(ARCH)/$(OUTPUT)
@@ -232,6 +244,15 @@ all: bin-$(ARCH)/$(OUTPUT)
 # Include header dependencies.
 -include $(HEADER_DEPS)
 
+# Bump the build number when any kernel object or userspace source is newer.
+$(BUILDNO_HDR): $(filter-out $(BUILDNO_OBJ),$(OBJ)) $(PROG_SRC)
+	../scripts/bump-build.sh $@
+
+# Force Syscall.cpp.o to recompile (and the kernel to relink) in the same pass
+# the header is bumped, so the embedded number always matches BuildNo.hpp even
+# on a clean build (where no .d dependency exists yet).
+$(BUILDNO_OBJ): $(BUILDNO_HDR)
+
 # Link rules for building the C compiler runtime.
 cc-runtime-$(ARCH)/cc-runtime.a: GNUmakefile cc-runtime/*
 	rm -rf cc-runtime-$(ARCH)