refactor: make encoding mechanisms version-free

[Xuanwo]

Part 4/12 of #7877. Depends on #8020.

This is an independently reviewable step toward the final layout demonstrated in #7979.

This PR separates reusable encoding mechanisms from file-version composition. lance-encoding exposes array-encoding, structural encoding, decoding, and compression mechanisms without selecting a file version. Exact V2.1, V2.2, and V2.3 modules in lance-file own their compression and encoding composition instead.

The goal is to make algorithm changes local and reusable while keeping the choice of algorithms isolated by exact file version. This removes the old previous encoding namespace and does not add a parallel compression abstraction.

Validation:

• cargo clippy --all --tests --benches -- -D warnings
• encoding benchmarks compile
• exact-version compatibility fixtures remain unchanged