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.

[e54-bot]

Independent ablation verification

Claim: OPY semantic lowering has real feature-local module ownership while preserving the existing lowering contract.

Authority: Issue #203 acceptance criteria, the existing OPY HIR/lowering contract, and the pinned differential corpus.

Surface: A fresh detached worktree was created from PR head 56ed342. The mod expressions; declaration was temporarily removed from crates/opy-rs/src/lower.rs. cargo check -p opy-rs then failed with exit 101 and repeated no method named lower_expr errors from the declaration, statement, and special-form consumers. The module declaration was restored, after which cargo test -p opy-rs --test differential passed (1/1) and cargo test -p opy-rs --test semantic_model passed (5/5).

Verdict: VERIFIED — the ablation breaks the feature-local ownership boundary, while the restored PR head passes independent corpus and semantic-model checks.

Limitations: This verifies the structural ownership claim and representative semantic compatibility; it is not a substitute for the full hosted CI suite, which is currently green.