docs: document custom property shape model

[Ramlaoui]

Summary

• fold the custom property scope and tensor shape contract into the existing architecture data-model page
• document molecule vs atom property ownership, value-level tensor shape, flat-batch constraints, and ASE ingestion rules in context
• remove the standalone dev/design page so tensor properties are not a disconnected feature note

Validation

• git diff --check
• uv run --project atompack-py --group docs --locked --no-sync sphinx-build -b html docs/source /tmp/atompack-tensor-docs-sphinx-html

Sphinx build succeeds with the existing missing-image warnings from performance.rst.