refactor: strip libgbm dependency and use sealed memfd for SHM fallback

[InternetOfTofu]

Replaces the GBM allocator in allocator.rs with a clean CPU-accessible memfd allocator for local placeholder mode (wl_shm). Applies file seals (F_SEAL_SHRINK, F_SEAL_GROW, F_SEAL_SEAL) to allocated memfd buffers to guarantee memory size immutability and prevent SIGBUS truncation bugs.

Removes all references to gbm BufferObjects across state and SHM handling, enabling out-of-the-box 100% statically linked builds (+crt-static).

Also updates wayland_codegen to emit &str instead of &String to resolve clippy warnings across protocol handlers.

TAG=agy
CONV=17ab6d56-57a3-4f9b-a034-5fdda38d047d

[InternetOfTofu]

still need to test that it works.

[InternetOfTofu]

(this is a prereq of #22)

[InternetOfTofu]

manual testing passed, local compositor pass through and in-vm compositor pass through both still work.

ldd is much clean and minimal:

	linux-vdso.so.1 
	libgcc_s.so.1
	libm.so.6
	libc.so.6 
	/lib64/ld-linux-x86-64.so.2