feat: FPU/SSE context save/restore for correct floating-point across context switches

feat: FPU/SSE context save/restore for correct floating-point across context switches

- arch_fpu_init(): initialize x87 FPU (CR0.NE, clear EM/TS), enable OSFXSR if FXSR supported
- arch_fpu_save/restore: FXSAVE/FXRSTOR (or FSAVE/FRSTOR fallback) per process
- FPU state (512B) added to struct process, initialized for new processes
- fork/clone inherit parent FPU state; kernel threads get clean state
- schedule() saves prev FPU state before context_switch, restores next after
- Heap header padded 8->16 bytes for 16-byte aligned kmalloc (FXSAVE requirement)
- Added -mno-sse -mno-mmx to kernel ARCH_CFLAGS (prevent SSE in kernel code)
- Weak stubs in src/kernel/fpu.c for non-x86 architectures