2026-09-01 (injector prototype & warning cleanup) - added injector_start prototype and cleaned unused consts

• src/injector/injector.h: Created header declaring int injector_start(payload_args_t *args); resolving -Wmissing-prototypes error.
• src/injector/injector.c: Included "injector/injector.h".
• src/injector/krw.c: Removed unused KERNEL_OFFSET_UCRED_CR_UID and KERNEL_OFFSET_UCRED_CR_PRISON resolving -Wunused-const-variable error.

Verified: All C files in src/injector/ and src/common/ compile cleanly under -Wall -Wextra -Werror -Wshadow -Wconversion -Wsign-conversion -Wstrict-prototypes -Wmissing-prototypes -Wvla.