refactor(lower): give semantic lowering feature-local ownership

[e54-bot]

Summary

• split declaration/symbol handling, statement/control-flow lowering, expression/builtin/member resolution, and OPY special forms into explicit internal modules
• keep shared Lowerer state and public lowering entry points unchanged
• preserve existing semantic, diagnostic, provenance, and Workshop ownership behavior

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-targets --all-features
• python3 -m unittest discover -s tools/overpy/tests

Fixes #203

[Teakowa]

LGTM.

Withdrawing approval because the required independent ablation could not be executed in the current environment; code review found no defect, but verification is incomplete until that check is run.