Add IntoIterator implementations for ProductType

[samrusani]

Description of Changes

Closes #1452.

• Implement consuming iteration for ProductType.
• Implement borrowed iteration for &ProductType.
• Use the new consuming iterator in the existing product-value generator and layout builder.
• Add a unit test covering both iterator forms.

API and ABI breaking changes

None. These are additive trait implementations with no runtime or ABI changes.

Expected complexity level and risk

1. This is a small, backward-compatible refactor localized to spacetimedb-sats.

Testing

• cargo fmt --all -- --check
• cargo test -p spacetimedb-sats
• cargo clippy -p spacetimedb-sats --all-targets -- -D warnings
• git diff --check

[CLAassistant]

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