refactor(wren-core): cube Phase C cleanup nits

[goldmedal]

Summary

Follow-up to PR #2265 — addresses the four CodeRabbit nits left unresolved at merge.

Nit 1+2 — tidy cube module exports (a00143f2)

• Moved pub use cube::{cube_query_to_sql, CubeQuery} to sit directly below pub mod cube; to match the file's convention.
• Narrowed cube to pub(crate) mod cube; so the curated mdl::{cube_query_to_sql, CubeQuery} surface is the single canonical path. Secondary DTOs (Granularity, CubeFilter, FilterValue, …) stay internal until a binding layer needs them.
• Verified wren-core-py and wren-core-wasm still compile — neither imports the cube DTOs.

Nit 3+4 — scope measure resolution + cache regex (725e6cb8)

• resolve_measures now walks the transitive closure of query.measures instead of every measure on the cube. A cycle among unrelated measures no longer fails an otherwise valid query.
• Pre-compute the word-boundary regex per measure name once and reuse it for both dependency discovery and the fixpoint substitution loop.
• Adds test_resolve_measures_ignores_unrequested_cycle covering the cycle case.

Test plan

• RUST_MIN_STACK=8388608 cargo test --lib --tests --bins — 117 tests pass (33 cube + 84 prior)
• cargo clippy --all-targets --all-features -- -D warnings — clean
• cargo fmt --all -- --check — clean
• cargo check -p wren-core-py and -p wren-core-wasm — clean
• CI on the wren-core path-filtered workflow passes

Closes the four CodeRabbit threads on #2265.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Enhanced query planning robustness to handle measure dependency cycles more gracefully. Queries now resolve dependencies only for explicitly requested measures, preventing unrequested dependency cycles from causing query planning failures.

• Tests

  • Added test coverage to validate proper handling of unrequested measure dependency cycles.

• Refactor

  • Optimized internal module organization and overall code structure.

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