feat(go): add embedded images (PNG/JPEG/GIF/BMP)

[Mukbeast4]

Summary

• Adds AddImage, AddImageFromBytes, GetImages, RemoveImages for embedding images anchored to cells
• Supports PNG, JPEG, GIF, BMP with magic-byte format detection
• Identical binaries are deduplicated via SHA1 in Pictures/<sha1>.<ext>
• Dynamic manifest generation for Pictures/* entries
• Full round-trip (save → open → read back) preserves cell ref, dimensions, offsets, binary data

Why

The library lacked image support, a common feature in spreadsheet libraries. Images are written as <draw:frame><draw:image xlink:href="Pictures/..."/></draw:frame> per the OpenDocument spec.

Test plan

• go test ./... (all 200+ tests pass)
• go vet ./...
• gofmt -s -l . (empty)
• 7 new tests: round-trip, file/bytes input, format detection, shared binary dedup, removal with ref counting, invalid format rejection
• Manual: open generated .ods in LibreOffice Calc to verify image renders at correct cell/size