fix(core): allow suboptimal_flops on mouth-audio smoothing lerp

[andymai]

Summary

Rust 1.96.0's clippy promoted clippy::suboptimal_flops to fire on the single-pole IIR smoothing step in mouth_from_audio.rs, which broke the Host (core + sim + drivers) CI gate on main. Because that gate is red on main, every open PR inherits the failure — all four open dependabot PRs (#586, #585, #584, #551) are blocked solely by this.

Clippy suggests f32::mul_add, but that lowers to libm::fmaf on the no_std Xtensa target — exactly the dependency this module is engineered to avoid (it sits beside the libm-free one_minus_exp_approx approximation). This matches the standing decision already documented in head_from_attention and lost_target_search, so the fix #[allow]s the lint with the same rationale rather than taking clippy's suggestion.

Test plan

• just check — fmt + workspace clippy (-D warnings) + host tests all green locally
• cargo clippy -p stackchan-core --all-targets --all-features -- -D warnings passes

[greptile-apps]

Greptile Summary

This PR suppresses the clippy::suboptimal_flops lint that Rust 1.96.0 promoted, which fired on the single-pole IIR smoothing step in mouth_from_audio.rs and broke CI for the entire codebase. The fix #[allow]s the lint with an inline reason, consistent with the identical suppress-and-explain pattern already used in lost_target_search.rs, head_from_attention.rs, and three other modules.

• Adds #[allow(clippy::suboptimal_flops, reason = "...")] to the lerp expression; the reason field is stable as of Rust 1.81 and this project's MSRV is 1.88.
• The original compound-assignment form is split into let next = …; self.current = next; to give the attribute a clean statement-level anchor — the math and semantics are unchanged.

Confidence Score: 5/5

Safe to merge — a one-line lint suppress with no semantic change to any computation.

The change is a targeted clippy allow attribute on an expression whose math is identical before and after. It replicates a pattern already established in six other sites in the same crate, the reason string matches lost_target_search.rs verbatim, and the reason field syntax is valid at the project's MSRV.

No files require special attention.

Important Files Changed

Filename	Overview
crates/stackchan-core/src/modifiers/mouth_from_audio.rs	Adds #[allow(clippy::suboptimal_flops)] with a reason string around the single-pole IIR lerp step, matching the identical pattern already in lost_target_search.rs, head_from_attention.rs, and several other modules; no functional change

_{Reviews (1): Last reviewed commit: "fix(core): allow suboptimal_flops on mou..." | Re-trigger Greptile}