Add on-console GameCube test suite + devkitPPC in the devcontainer

gc_bba_test/ builds a .dol that drives the adapter over raw EXI from the
console and reports pass/fail per test on the TV, navigable with the pad.

Uses libogc's EXI API against EXI0:2 (Serial Port 1) rather than its bba
network driver: driving the register file directly gives each check its own
verdict, so a failure points at a specific register instead of at
'networking'.  Six suites - identity, registers, reset/control, EXI link
integrity (including an EXI clock-index sweep), ethernet TX/RX, SPRAM/DMA
streaming.

Encodes the hardware lessons from bring-up so the app cannot mislead:
  * the device ID is a one-shot cleared by the first NCRA reset, so it is
    reported SKIP on a re-run rather than FAIL
  * TI means the frame reached the W5100, NOT the wire - the TX test says so
  * register reads clock exactly the requested length (over-clocking
    desynchronises the next transaction)
  * TXDATA is written one byte per transaction (eff_addr auto-increments)

.devcontainer/Dockerfile gains the devkitPro gamecube-dev group.  devkitPro
publishes no .deb on GitHub, so this uses their installer script against
apt.devkitpro.org -- which is 403 from inside the running container's egress
but reachable from a host-side docker build.  Rebuild the container to get it.

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

gc_bba_test/Makefile

@@ -0,0 +1,71 @@
+#---------------------------------------------------------------------------------
+# re-bba-rb on-console test suite — GameCube (.dol), devkitPPC + libogc
+#
+#   make            build gc_bba_test.dol
+#   make clean
+#
+# Needs devkitPro with the `gamecube-dev` group installed (DEVKITPPC set).
+# Run the .dol from Swiss (SD Gecko / SD2SP2 / USB Gecko) on a console with the
+# re-bba-rb adapter in Serial Port 1.
+#---------------------------------------------------------------------------------
+ifeq ($(strip $(DEVKITPPC)),)
+$(error "DEVKITPPC is not set. Install devkitPro's gamecube-dev group, or rebuild the devcontainer.")
+endif
+
+include $(DEVKITPPC)/gamecube_rules
+
+TARGET		:=	$(notdir $(CURDIR))
+BUILD		:=	build
+SOURCES		:=	source
+DATA		:=	data
+INCLUDES	:=
+
+CFLAGS	:= -g -O2 -Wall -Wextra $(MACHDEP) $(INCLUDE)
+CXXFLAGS:= $(CFLAGS)
+LDFLAGS	:= -g $(MACHDEP) -Wl,-Map,$(notdir $@).map
+
+LIBS	:= -logc -lm
+LIBDIRS	:=
+
+#---------------------------------------------------------------------------------
+ifneq ($(BUILD),$(notdir $(CURDIR)))
+#---------------------------------------------------------------------------------
+export OUTPUT	:=	$(CURDIR)/$(TARGET)
+export VPATH	:=	$(foreach dir,$(SOURCES),$(CURDIR)/$(dir)) \
+					$(foreach dir,$(DATA),$(CURDIR)/$(dir))
+export DEPSDIR	:=	$(CURDIR)/$(BUILD)
+
+CFILES		:=	$(foreach dir,$(SOURCES),$(notdir $(wildcard $(dir)/*.c)))
+CXXFILES	:=	$(foreach dir,$(SOURCES),$(notdir $(wildcard $(dir)/*.cpp)))
+sFILES		:=	$(foreach dir,$(SOURCES),$(notdir $(wildcard $(dir)/*.s)))
+SFILES		:=	$(foreach dir,$(SOURCES),$(notdir $(wildcard $(dir)/*.S)))
+
+export LD	:=	$(CC)
+export OFILES	:=	$(CFILES:.c=.o) $(CXXFILES:.cpp=.o) $(sFILES:.s=.o) $(SFILES:.S=.o)
+export INCLUDE	:=	$(foreach dir,$(INCLUDES),-I$(CURDIR)/$(dir)) \
+					$(foreach dir,$(LIBDIRS),-I$(dir)/include) \
+					-I$(CURDIR)/$(BUILD) -I$(LIBOGC_INC)
+export LIBPATHS	:=	$(foreach dir,$(LIBDIRS),-L$(dir)/lib) -L$(LIBOGC_LIB)
+
+.PHONY: $(BUILD) clean
+
+$(BUILD):
+	@[ -d $@ ] || mkdir -p $@
+	@$(MAKE) --no-print-directory -C $(BUILD) -f $(CURDIR)/Makefile
+
+clean:
+	@echo clean ...
+	@rm -fr $(BUILD) $(OUTPUT).elf $(OUTPUT).dol $(OUTPUT).dol.map
+
+#---------------------------------------------------------------------------------
+else
+
+DEPENDS	:= $(OFILES:.o=.d)
+
+$(OUTPUT).dol: $(OUTPUT).elf
+$(OUTPUT).elf: $(OFILES)
+
+-include $(DEPENDS)
+
+endif
+#---------------------------------------------------------------------------------