chore(tui): gate audited test-only helpers

[wuisabel-gif]

Summary

Addresses the approved bounded first slice of #5587.

This PR converts 13 audited test-only helper surfaces from #[allow(dead_code)] to #[cfg(test)] across TUI rendering/text fixtures:

• whale gallery constants and text/ASCII render helpers;
• focus-texture accounting assertion;
• file-tree scroll test helper;
• ambient frame-budget assertion constant;
• history exploration/generic render convenience wrappers;
• reasoning-summary test helper.

Where a gated helper had no existing direct consumer, a small property test now exercises the helper (FileTreeState::adjust_scroll, ExploringCell::lines_with_motion, and whale state priority). Production consumers were verified with cargo check -p codewhale-tui --lib; no runtime behavior or production code path changes are intended.

Stale production-use allowances, blanket conversions, and Tier C intent-check items remain out of scope.

Tests

Passed locally:

• cargo fmt --all -- --check
• git diff --check
• affected module suites: whales (14), focus texture (9), file tree (8), history (41), ambient life (29)
• RUST_MIN_STACK=16777216 cargo test -p codewhale-tui --lib --locked — 11,228 passed, 0 failed, 13 ignored
• cargo check -p codewhale-tui --lib --locked

The default-stack full TUI test run is blocked by the known #5585 baseline stack overflow in runtime_api::tests::reload_config_preserves_profile_selected_named_custom_route; the documented 16 MiB stack setting passes the full suite.

Strict Clippy is currently blocked on the untouched runtime_threads.rs:2562 baseline function accumulate_runtime_cost_coverage (clippy::too_many_arguments, 9/7), which is outside this cleanup slice and already present on origin/main.

No provider credentials or network access are required.

No-Issue: This PR addresses the approved first #5587 test-only-marker slice without automatically closing the broader audit issue.

[Hmbown]

Thank you, Isabel. I verified this patch is already on main through #5667 (6ef5f87d8182eb38a18cfff99e58bbf7ef01b20c). Original 8a27b8c57 and landed ab4c8a8e1 have the same stable patch ID, and the landed commit preserves Isabel Wu <231155141+wuisabel-gif@users.noreply.github.com> as author.

I’m closing this duplicate PR because the contribution is already shipped on main; merging this stale head again would duplicate the patch.