Fix prefetched scalar decoding precedence

[XX-Q]

Fixes #1804.

Prefetched association elements were always decoded through Decodable, even when their database-specific conformances should take precedence. This made values such as UUID-backed DatabaseValueConvertible types fail when fetched from a single selected column.

This change:

• prefers FetchableRecord.init(row:) over Decodable for prefetched rows;
• uses the existing ColumnDecoder paths for Data, Date, DatabaseValueConvertible, and StatementColumnConvertible values;
• keeps ordinary row-based Decodable decoding as the fallback;
• adds regression coverage for both precedence rules.

Validation:

• swift build passes.
• A standalone reproduction of the UUID/MedalID case now passes.
• A standalone reproduction confirms FetchableRecord.init(row:) is selected over Decodable.
• The XCTest suite could not be run locally because this machine only has Command Line Tools and its Swift toolchain reports no such module XCTest; the added tests are intended to run in CI.

Pull Request Checklist

• CONTRIBUTING: I have read the contribution guidelines.
• BRANCH: This pull request targets development.
• DOCUMENTATION: Inline documentation has been updated. (Not applicable: internal bug fix with no API change.)
• DOCUMENTATION: README.md or another dedicated guide has been updated. (Not applicable: internal bug fix with no API change.)
• TESTS: Changes are tested with regression tests and standalone reproductions.
• TESTS: make smokeTest runs without failure. (Not run: full Xcode is unavailable locally.)