REQ-GIT-083: Delta-base reads return stored object bytes rather than a parse and re-encode round trip

Callers that need an object's raw bytes — thin-pack REF_DELTA bases above all — read them directly instead of decoding a struct and re-encoding it. Delta application has no expected SHA to verify the reconstruction against: the stored SHA is computed from the delta result, so a base that is not byte-identical to the original would be stored as a corrupted object under a self-consistent but wrong SHA rather than failing loudly. The raw read applies the same size cap and SHA verification as the decoding read.