build: T= aliases ARCH= (board/SBC selection), lock StarForth to fastest

T= is now shorthand for ARCH=, selecting the target board (raspi, amd64,
aarch64, riscv64). StarForth TARGET is locked to fastest — it's the only
build profile. Makefile.starkernel T= wired the same way before ARCH
normalization runs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Makefile.starkernel

@@ -4,10 +4,9 @@
 #
 # Quick start:
 #   make -f Makefile.starkernel                        — build for host arch
-#   make -f Makefile.starkernel ARCH=amd64 qemu        — build + boot (x86_64)
-#   make -f Makefile.starkernel ARCH=aarch64 qemu      — build + boot (aarch64)
-#   make -f Makefile.starkernel ARCH=riscv64 qemu      — build + boot (riscv64)
-#   make -f Makefile.starkernel ARCH=amd64 T=kernel    — T= is a shorthand for TARGET=
+#   make -f Makefile.starkernel T=amd64 qemu           — build + boot (x86_64)   T= is shorthand for ARCH=
+#   make -f Makefile.starkernel T=aarch64 qemu         — build + boot (aarch64)
+#   make -f Makefile.starkernel T=riscv64 qemu         — build + boot (riscv64)
 #   make -f Makefile.starkernel help                   — show all targets
 # ==============================================================================
 
@@ -27,6 +26,10 @@ canon_arch = $(strip \
     $(if $(filter riscv riscv64,$1),riscv64,$1))))))
 
 UNAME_M        := $(shell uname -m)
+# T= is a shorthand for ARCH= (board/SBC selection)
+ifdef T
+ARCH           ?= $(T)
+endif
 ARCH           ?= $(call canon_arch,$(UNAME_M))
 override ARCH  := $(call canon_arch,$(ARCH))
 
@@ -43,11 +46,8 @@ endif
 KCONFIG_ARCH_DIR := $(ARCH)
 include mk/Kconfig.mk
 
-# Target type (default: kernel). T= is a shorthand alias.
+# Target type (default: kernel)
 TARGET    ?= kernel
-ifdef T
-TARGET    := $(T)
-endif
 
 # Kernel-only flags (Phase 6): repointed through the Kconfig bridge, same
 # drop-in-replacement-for-`?=` behavior as every other family in this