Add Chunk::object_offsets() read-only accessor

[gpradofe]

Chunk already exposes refs() to iterate the top-level indirect object references in order, and as_bytes() to look at the raw buffer. The byte offset of each object is already tracked internally (pub(crate) offsets: Vec<(Ref, usize)>) but is not exposed.

This PR adds a small object_offsets() accessor that returns the (Ref, byte_offset) pairs alongside the byte buffer, which is useful when:

• Doing piecewise / partial serialization (e.g. writing a large PDF in multiple passes and needing to recover where each object sits inside a chunk).
• Indexing into as_bytes() directly from an external object store or cache that wants to record object positions alongside the bytes.

The accessor is read-only and exposes data that's already derivable from the public API plus a linear scan; it doesn't change any invariants.

Test plan

• Added test_chunk_object_offsets verifying the returned offsets match refs() and that each offset points at a real N 0 obj header inside the chunk's byte buffer.
• cargo test passes.
• cargo clippy --all-targets --all-features clean under -Dwarnings.
• cargo fmt --check clean.