fix(xlsx): retain embedded worksheet images

[Fanfulla]

This fixes the XLSX side of #139.

The parser now follows the worksheet -> drawing -> image relationship chain and stores embedded image bytes in Document.assets, using the existing AssetSink behavior for byte limits and deduplication. Each asset keeps its media type and original package path.

I kept Markdown output out of scope. This change only makes the shared document model complete for XLSX files. Positional Markdown references are still tracked in #63.

Tests:

• added a regression test with a minimal XLSX containing an embedded image
• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --locked

Fixes #139

[cubic-dev-ai]

All reported issues were addressed across 1 file

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[Fanfulla]

Fixed in 11c8c14. rel_target_bytes and rel_image_source now accept mutable references directly, so XLSX, DOCX, and PPTX use the same relationship resolution and asset retention path.

I reran cargo fmt --all --check, Clippy with warnings denied, and the locked test suite.