Improve README presentation

[shaypal5]

Summary

• Rework the README into a clearer first-visit gateway for the hletterscript seed corpus.
• Add badges, exact corpus counts, pipeline positioning, dataset layout, licensing, validation, and contribution guidance.
• Add a real sample grid generated from the indexed crop PNGs so visitors can immediately see the dataset shape.
• Add the required creator attribution near the top and in a bottom Credits section.
• Pin the upstream HASH checkout used by CI to the latest referenced HASH commit so cross-validation stays reproducible.
• Update repository metadata description and topics to match the project purpose.

Validation

• python3 scripts/validate_indexes.py
• python3 scripts/validate_indexes.py --upstream-path <temporary HASH worktree at 0e162a78d609a47858c460431be3d689b2e9e31e>
• python3 scripts/generate_release_artifacts.py --check
• python3 -m pytest
• git diff --check
• README smoke check for exactly two creator mentions and bottom Credits placement

Notes

No open GitHub milestones are available for this repository, so no milestone was assigned.