Exclude large previews from crate package

[newjordan]

Summary

• Exclude the generated loader sheet HTML from the published crate archive.
• Exclude README gallery PNGs from the published crate archive while keeping them in the repository.

Validation

• cargo publish --dry-run
• Package size after exclusions: 208 files, 31.2MiB unpacked / 1.7MiB compressed

Context

• crates.io rejected the first 0.1.8 publish attempt with 413 Payload Too Large at 11.7MiB compressed.