Integrate Mahony attitude filter into Rev2 firmware

[bjornhbengtsson]

Description

Updates the Rev2 flight-computer firmware build configuration to include the Mahony attitude filter added in the mod repository.

Changes include:

• Added mod/mahony/mahony.c to the Rev2 firmware source list
• Added mod/mahony to the compiler include paths so mahony.h is available
• Updated the mod submodule reference to include the Mahony implementation and sensor-state integration

Assisted by ChatGPT 5.6

Dependency

Depends on SunDevilRocketry/mod#142 (comment)

This PR is based on feature/gravity-compensated-acceleration so that it follows the same dependency order as the corresponding mod changes.

Testing

• Rev2 firmware builds successfully with arm-none-eabi-gcc
• mod/mahony/mahony.c compiles into build/mahony.o
• Mahony object is included in the final firmware link
• Generated appa.elf, appa.hex, and appa.bin
• Working tree remains clean after the build

Other

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Reviewer Checklist

Standards

• Follows FCF Architectural Standards
• Follows SDR Coding Standards
• Code complexity/function Size is minimized
• Code is testable
• Code is readable and commented properly
• License terms are respected

Accuracy

• Code implements the correct requirement (a.k.a. does the right thing)
• Code is logically correct (a.k.a. does the thing right)

Error Handling

• Potentially unsafe functions return a status code
• Error returns properly handled
• Fail-fast errors are only thrown when unsafe to continue software execution
• Debug errors are thrown for exceptions where execution should still continue (to be noticed during development)

Memory

• Stack allocated memory is scoped correctly
• Heap allocated memory is not used
• Statically/Globally allocated memory is minimized except when necessary
• Pointers are used correctly
• Concurrent access has been considered (especially by/from interrupt service routines)

Performance

• Rate limiters are respected
• Busy waiting is avoided in performance sensitive code
• "Delay" calls are not used in performance sensitive code
• If performance is negatively impacted, a justification is provided and the impact is quantified