docs: follow up GP import doc coverage

[PhysShell]

Motivation

• Code coverage tooling flagged missing docstrings on private Guitar Pro import helpers and a Clippy nursery lint surfaced during verification.
• Small follow-up changes were needed to satisfy the project's documentation and linting gates without changing behaviour.

Description

• Added Rust doc comments to the Guitar Pro import helpers in core/src/gp.rs including gp_song_to_score, build_gp_master_bars, build_gp_track, build_gp_voice, append_beat, and rest_group to improve doc coverage.
• Rewrote the accumulator updates in rotated_correlation (core/src/complement.rs) to use f64::mul_add for numerator, var_x, and var_y, addressing the clippy::suboptimal_flops nursery lint.
• Committed the changes on the branch and prepared this follow-up PR; no public API or schema changes were made.

Testing

• Ran cargo fmt --all --check which succeeded.
• Ran cargo clippy --workspace --all-targets -- -D warnings which succeeded with the nursery lint fixed.
• Ran the full test suite via cargo test --workspace which completed with all tests passing.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve exact-tie key ordering

When a part's duration-weighted pitch-class histogram gives an exact tie between two rotated KK profiles, the documented tie-breaker is the first key scanned. The fused accumulation can split that exact tie by one ulp: for example histogram [194,498,821,804,875,237,243,964,12,952,316,475] makes D major and G major exact ties, but this line rounds D's numerator lower and estimate_harmony now picks G major, changing PartProfile::harmony and downstream complement pitch material for existing ambiguous parts. Either keep the previous accumulation or add an explicit epsilon/tie comparison so lint cleanup doesn't alter key ordering.

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