Add simple ODB helper and document alternates usage

[ulrik-s]

Summary

• add a simple-odb helper that prepares a loose-object directory and stores blobs using Git's hashing and compression helpers
• expose test-tool plumbing plus t/t0039-simple-odb.sh to exercise reading those objects through the alternates mechanism
• document how the helper leverages the existing alternates interface to participate in object lookup

Testing

• make -j4
• (cd t && ./t0039-simple-odb.sh)

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https://chatgpt.com/codex/tasks/task_e_68c3c338718883249dcd24718d503c88