Add header dependency tracking to the Makefile

[pangin]

Bug

The object pattern rule compiles without generating dependency files, so make only knows that each .o depends on its .cpp. Editing a header rebuilds nothing that includes it.

Symptom

After changing a class layout in a header (e.g. adding a member), an incremental build links stale objects compiled against the old layout together with fresh ones using the new layout. The result is silent memory corruption at runtime — we hit it as "stack smashing detected" and random segfaults while porting FastCarPlay to a Raspberry Pi Zero W (ARMv6) head unit, and it was very hard to trace back to the build system. make clean "fixes" it, which is the tell.

Fix

Compile with -MMD -MP so the compiler emits a .d makefile per object listing every header it includes, and -include $(OBJS:.o=.d) to feed those back into make. Touching a header now rebuilds exactly the objects that include it. First builds are unaffected; the .d files live in the existing per-build-type object tree and are removed by clean.

🤖 Generated with Claude Code