Fix generation of railway tunnels

[floze-the-genius]

Summary

• route railway=* ways tagged tunnel=yes through the existing two-phase underground railway generator instead of dropping them
• generalize the subway-only tunnel bookkeeping names to cover ordinary train tunnels
• add regression coverage that verifies a heavy-rail tunnel keeps its rail, carves its interior, preserves its walls, and does not render at grade

Fixes #994

Testing

• cargo fmt --all -- --check
• cargo test --all-targets --all-features (359 passed, 6 ignored)
• cargo clippy --all-targets --all-features -- -D warnings

[chatgpt-codex-connector]

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[floze-the-genius]

The benchmark itself completed successfully: generation time was 19s versus the 18s baseline, and peak memory was 1181 MB versus 1160 MB; both are classified as unchanged by the workflow. The check is red only because the fork-scoped GITHUB_TOKEN could not create the benchmark report comment (Resource not accessible by integration), so I am recording the result here manually.