Validate encrypted tile payload bounds; add regression test and update docs

[AEliu]

Motivation

• Prevent malformed encrypted tile payloads from being silently accepted or causing subtle errors during tile download by adding defensive validation.
• Keep documentation aligned with the current package layout and fix a small wording typo.
• Add a regression test to ensure encrypted tile validation remains enforced.

Description

• Add bounds checks in decrypt_tile_if_needed to validate header metadata offsets and encrypted payload length and raise DownloadError on malformed data.
• Add test_download_tiles_rejects_malformed_encrypted_payload in tests/test_tile_cache.py to cover malformed encrypted tile handling during tile download.
• Update docs/project-status.md to reference src/artx/... instead of stale src/googleart_download/... and change compatibility-shell to compatibility-shim.

Testing

• Ran uv run pytest -q tests/test_tile_cache.py tests/test_transport.py and received 32 passed.
• Ran uv run ruff check src/artx/download/tiles.py tests/test_tile_cache.py docs/project-status.md and all checks passed.

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Codex Task