[andino_firmware] Scope compiler warning flags to project source and add stricter checks

[jballoffet]

🎉 New feature

Summary

This PR splits build_flags into build_flags (just -I include) and build_src_flags, so warning enforcement only applies to our own src/ code and not to frameworks/libraries we don't control. It also adds -Werror so warnings fail the build, scoped to build_src_flags, and adds -Woverloaded-virtual, -Wcast-qual, -Wuseless-cast, and -Wnull-dereference for additional static safety.

Test it

docker compose -f docker/compose.yaml run --rm dev pio run -e uno
Container andino-firmware-dev-run-e2b330fe2cbd Creating 
Container andino-firmware-dev-run-e2b330fe2cbd Created 
Processing uno (board: uno; platform: atmelavr; framework: arduino)
-----------------------------------------------------------------------------------------------------------------------------------------------
Verbose mode can be enabled via `-v, --verbose` option
CONFIGURATION: https://docs.platformio.org/page/boards/atmelavr/uno.html
PLATFORM: Atmel AVR (5.3.0) > Arduino Uno
HARDWARE: ATMEGA328P 16MHz, 2KB RAM, 31.50KB Flash
DEBUG: Current (avr-stub) External (avr-stub, simavr)
PACKAGES: 
 - framework-arduino-avr @ 5.4.0 
 - toolchain-atmelavr @ 1.70300.191015 (7.3.0)
LDF: Library Dependency Finder -> https://bit.ly/configure-pio-ldf
LDF Modes: Finder ~ chain, Compatibility ~ soft
Found 8 compatible libraries
Scanning dependencies...
Dependency Graph
|-- Wire @ 1.0
|-- SPI @ 1.0
|-- Adafruit BNO055 @ 1.6.4
|-- Adafruit BusIO @ 1.17.4
|-- Adafruit Unified Sensor @ 1.1.15
Building in release mode
Checking size .pio/build/uno/firmware.elf
Advanced Memory Usage is available via "PlatformIO Home > Project Inspect"
RAM:   [=====     ]  51.5% (used 1055 bytes from 2048 bytes)
Flash: [=====     ]  51.2% (used 16502 bytes from 32256 bytes)
========================================================= [SUCCESS] Took 0.50 seconds =========================================================

Environment    Status    Duration
-------------  --------  ------------
uno            SUCCESS   00:00:00.502
========================================================= 1 succeeded in 00:00:00.502 =========================================================

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.