Add inventory tracker contract example

[alexgduarte]

Contributes to #11

Summary

• Add a new inventory-tracker simple contract example.
• Let each address manage its own inventory records with sequential per-user item IDs.
• Support item creation, metadata updates, quantity setting, quantity increases/decreases, deletion, per-user counts, and paginated item listing.
• Include safe quantity underflow handling, owner-managed maximum item limits, ownership transfer, input validation, README documentation, schema generation, and root README linking.
• Include eight focused unit tests covering config, listing, metadata updates, quantity accounting, ownership isolation, limits, validation, deletion, and owner controls.

Validation

• cargo fmt --manifest-path inventory-tracker/Cargo.toml -- --check
• cargo test --manifest-path inventory-tracker/Cargo.toml --target-dir /private/tmp/codex-inventory-tracker-target
• git diff --check