build(mcu): vendor STM32F7 HAL + gcc Makefile (PR 1 of firmware bring-up)

First of three planned PRs to produce a bootable STM32F746ZGT7 firmware
image from the sources already in-tree. The original CubeIDE project
(.ioc + generated Core/Drivers/Middlewares) never made it into the repo,
so the build is being reconstructed from scratch against the vendored HAL.

Scope of this PR:
  - Pin arm-none-eabi-gcc 15 (brew --cask gcc-arm-embedded) as the
    toolchain for local macOS builds.
  - Vendor trimmed copies of the four upstream trees needed by a minimal
    STM32F7 project:
      * CMSIS_5 5.9.0          Cortex-M7 Core/Include subset (256 KB)
      * cmsis_device_f7 v1.2.8  F746-only device header + startup (1 MB)
      * stm32f7xx_hal_driver v1.3.1  full HAL Inc/Src (9.3 MB)
      * stm32_mw_usb_device v2.11.3  Core + Class/CDC (200 KB)
    Total vendor tree: 11 MB, 228 files. VERSION files pin each drop.
  - Add Makefile targeting -mcpu=cortex-m7 -mfpu=fpv5-sp-d16 -mfloat-abi=hard.
    `make compile` produces 150 objects covering:
      * all enabled HAL modules (GPIO/EXTI/DMA/RCC/FLASH/PWR/I2C/CORTEX/
        IWDG/SPI/TIM/UART/PCD per stm32f7xx_hal_conf.h)
      * the no-os ADI middleware layer
      * ADAR1000/ADF4382/AD9523/BMP180/GY-85/TinyGPS++ drivers
      * um982_gps, platform_noos_stm32, system_stm32f7xx, startup
    Deliberately excluded (need PR 2 Cube glue):
      * main.cpp / gps_handler.cpp   — require usb_device.h, usbd_cdc_if.h
      * USB Device Library sources   — require user-provided usbd_conf.h
      * *_template.c                 — ST user-copy stubs, never compiled

Incidental fixes folded in (these are hard blockers, not cosmetic):
  - Delete 9_1_1_C_Cpp_Libraries/errno.h.  It was an ADI armcc-only shim
    whose EINVAL/ENOENT/etc. macros are gated on __ARMCC_VERSION and
    relies on `#include_next <errno.h>`, which gcc refuses when the file
    was found via quoted-include in the source directory.  With the shim
    gone every `#include "errno.h"` / `#include <errno.h>` now resolves
    straight to newlib's, which is what every other TU in the tree
    already assumed.
  - Makefile demotes four gcc-15 defaults back to warnings
    (-Wno-error=incompatible-pointer-types/discarded-qualifiers/
    int-conversion/implicit-function-declaration).  gcc 14 promoted
    these to errors by default; the underlying diagnostics are
    pre-existing type mismatches in the ADI no-os layer (e.g. const
    qualifier dropped in stm32_dma.c:281 function-pointer assignment),
    not new regressions.  Real fixes are out of scope for PR 1.

Still to come:
  PR 2 — hand-written Cube glue: STM32F746ZGTx_FLASH.ld, stm32f7xx_it.c,
         stm32f7xx_hal_msp.c, SystemClock_Config, MX_*_Init for the
         peripherals in main.cpp, usbd_conf.{h,c}, usbd_desc.c,
         usbd_cdc_if.c, and a C main() shim that calls into app_main().
         Closes the link; produces firmware.elf/.bin/.hex.
  PR 3 — hardware bring-up: flash, iterate on HSE/PLL + DMA + SPI mode
         against the real board until the existing firmware behaviour
         is restored.

9_Firmware/9_1_Microcontroller/Makefile

@@ -0,0 +1,139 @@
+# ============================================================================
+# AERIS-10 STM32F746ZGT7 firmware build (PR 1: compile-only)
+# ============================================================================
+# Part:      STM32F746ZGT7 (Cortex-M7, 1 MB flash, 320 KB RAM, 216 MHz, LQFP144)
+# Toolchain: arm-none-eabi-gcc 15.x (brew --cask gcc-arm-embedded)
+#
+# PR 1 scope: compile every source file to an object.  Linking is expected to
+# fail because STM32CubeIDE-generated glue (main.c entry, SystemClock_Config,
+# MX_*_Init, stm32f7xx_it.c, stm32f7xx_hal_msp.c, linker script, USB CDC
+# glue) is missing.  PR 2 will hand-write those files and close the link.
+#
+# Targets:
+#   make              -- compile all .c/.cpp/.s to objects (default)
+#   make compile      -- same
+#   make link         -- attempt to link (WILL FAIL in PR 1; left here so the
+#                        missing-symbol list drives PR 2 scope)
+#   make clean        -- remove build/
+# ============================================================================
+
+# ---- toolchain -------------------------------------------------------------
+PREFIX  := arm-none-eabi-
+CC      := $(PREFIX)gcc
+CXX     := $(PREFIX)g++
+AS      := $(PREFIX)gcc -x assembler-with-cpp
+LD      := $(PREFIX)g++
+OBJCOPY := $(PREFIX)objcopy
+SIZE    := $(PREFIX)size
+
+# ---- tree ------------------------------------------------------------------
+BUILD   := build
+OBJDIR  := $(BUILD)/obj
+
+VENDOR  := vendor
+APP     := 9_1_3_C_Cpp_Code
+LIB     := 9_1_1_C_Cpp_Libraries
+
+# ---- target part + CPU flags ----------------------------------------------
+MCU     := -mcpu=cortex-m7 -mthumb -mfpu=fpv5-sp-d16 -mfloat-abi=hard
+DEFS    := -DSTM32F746xx -DUSE_HAL_DRIVER -DARM_MATH_CM7 -D__FPU_PRESENT=1
+
+# ---- include paths (order matters; app overrides vendor on ambiguity) -----
+INC := \
+  -I$(APP) \
+  -I$(LIB) \
+  -I$(VENDOR)/cmsis_core/Include \
+  -I$(VENDOR)/cmsis_device_f7/Include \
+  -I$(VENDOR)/stm32f7xx_hal_driver/Inc \
+  -I$(VENDOR)/stm32_mw_usb_device/Core/Inc \
+  -I$(VENDOR)/stm32_mw_usb_device/Class/CDC/Inc
+
+# ---- warnings / optimisation ----------------------------------------------
+OPT      := -Og -g3
+WARN     := -Wall -Wextra -Wno-unused-parameter -Wno-unused-function \
+            -Wno-missing-field-initializers \
+            -Wno-error=incompatible-pointer-types \
+            -Wno-error=discarded-qualifiers \
+            -Wno-error=int-conversion \
+            -Wno-error=implicit-function-declaration
+COMMON   := $(MCU) $(DEFS) $(INC) $(OPT) $(WARN) -ffunction-sections -fdata-sections -fno-common
+
+CFLAGS   := $(COMMON) -std=gnu11
+CXXFLAGS := $(COMMON) -std=gnu++17 -fno-exceptions -fno-rtti -fno-threadsafe-statics
+ASFLAGS  := $(MCU) $(DEFS) -g3
+
+# ---- sources ---------------------------------------------------------------
+# Application C/C++
+# main.cpp needs Cube-generated usb_device.h / usbd_cdc_if.h — PR 2 target.
+# Exclude from PR 1 compile; um982_gps.c/.h compile cleanly and stay in.
+APP_C       := $(wildcard $(APP)/*.c)
+APP_CPP_ALL := $(wildcard $(APP)/*.cpp)
+APP_CPP     := $(filter-out $(APP)/main.cpp, $(APP_CPP_ALL))
+LIB_C   := $(wildcard $(LIB)/*.c)
+# gps_handler.cpp includes usb_device.h (Cube glue) — deferred to PR 2
+LIB_CPP := $(filter-out $(LIB)/gps_handler.cpp, $(wildcard $(LIB)/*.cpp))
+
+# Vendor: all enabled HAL modules compile cleanly because each _hal_<x>.c is
+# guarded by `#ifdef HAL_<X>_MODULE_ENABLED` in stm32f7xx_hal_conf.h.
+# Build every .c in Src/ and let the guard do the filtering.
+# Exclude *_template.c — ST ships these as user-copy stubs, not compile targets
+HAL_C   := $(filter-out %_template.c, $(wildcard $(VENDOR)/stm32f7xx_hal_driver/Src/*.c))
+# USB Device Library requires a user-written usbd_conf.h (Cube glue, PR 2).
+# PR 1 leaves this empty so the vendor tree compiles without that config.
+USB_C   :=
+SYS_C   := $(VENDOR)/cmsis_device_f7/Source/Templates/system_stm32f7xx.c
+STARTUP := $(VENDOR)/cmsis_device_f7/Source/Templates/gcc/startup_stm32f746xx.s
+
+SRCS_C   := $(APP_C) $(LIB_C) $(HAL_C) $(USB_C) $(SYS_C)
+SRCS_CPP := $(APP_CPP) $(LIB_CPP)
+SRCS_S   := $(STARTUP)
+
+OBJS := \
+  $(patsubst %.c,$(OBJDIR)/%.o,$(SRCS_C)) \
+  $(patsubst %.cpp,$(OBJDIR)/%.o,$(SRCS_CPP)) \
+  $(patsubst %.s,$(OBJDIR)/%.o,$(SRCS_S))
+
+DEPS := $(OBJS:.o=.d)
+
+# ---- link (PR 2) -----------------------------------------------------------
+# Linker script is hand-written in PR 2 at linker/STM32F746ZGTx_FLASH.ld
+LDSCRIPT := linker/STM32F746ZGTx_FLASH.ld
+LDFLAGS  := $(MCU) -T$(LDSCRIPT) -Wl,--gc-sections -Wl,-Map=$(BUILD)/firmware.map \
+            -specs=nano.specs -specs=nosys.specs -static
+
+ELF := $(BUILD)/firmware.elf
+BIN := $(BUILD)/firmware.bin
+HEX := $(BUILD)/firmware.hex
+
+# ---- rules -----------------------------------------------------------------
+.PHONY: all compile link clean size
+
+all: compile
+
+compile: $(OBJS)
+	@echo "compiled $(words $(OBJS)) objects"
+
+link: $(ELF)
+
+$(ELF): $(OBJS) $(LDSCRIPT)
+	$(LD) $(LDFLAGS) $(OBJS) -o $@ -lm
+	$(SIZE) $@
+	$(OBJCOPY) -O binary $@ $(BIN)
+	$(OBJCOPY) -O ihex   $@ $(HEX)
+
+$(OBJDIR)/%.o: %.c
+	@mkdir -p $(dir $@)
+	$(CC) $(CFLAGS) -MMD -MP -c $< -o $@
+
+$(OBJDIR)/%.o: %.cpp
+	@mkdir -p $(dir $@)
+	$(CXX) $(CXXFLAGS) -MMD -MP -c $< -o $@
+
+$(OBJDIR)/%.o: %.s
+	@mkdir -p $(dir $@)
+	$(AS) $(ASFLAGS) -c $< -o $@
+
+clean:
+	rm -rf $(BUILD)
+
+-include $(DEPS)