feat: expose TIFF.header_byte_size for prefetch sizing

[kylebarron]

Summary

• Adds TIFF::header_byte_size() (Rust) and TIFF.header_byte_size (Python) returning the minimum non-zero offset across every IFD's TileOffsets and StripOffsets.
• For a typical COG, this is the smallest prefetch= value that lets a future open complete metadata reading in a single request.
• Pure function over already-parsed IFDs — no changes to the metadata reader, fetch, or IFD types.

Closes #301.

Test plan

• New Rust test asserts the accessor matches the min-offset computation on an existing fixture.
• New Python test mirrors the assertion through TIFF.open on the eox fixture.
• `cargo test --all-features` — all 56 + 6 doctests pass.
• `pytest` (python) — all 43 tests pass.

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