Adopt Cargo 1.97 warning policy and close toolchain audit gaps

[acgetchell]

Summary

Complete the Rust/Cargo 1.97 tooling audit that followed the MSRV upgrade in #174 and PR #183. Evaluate Cargo's new cache-friendly warning policy, explicitly close the remaining symbol-mangling and linker-message questions, and record no-change conclusions for unrelated 1.97 features.

This is post-v0.4.4 tooling follow-up. It does not change the published API, numerical behavior, MSRV, or v0.4.4 artifacts, and it does not require an immediate patch release.

Current state

• Cargo.toml already denies Rust warnings through [lints.rust].
• Benchmark compilation in justfile still uses RUSTFLAGS='-D warnings', which can invalidate Cargo build caches.
• Documentation builds use RUSTDOCFLAGS='-D warnings'; Cargo's new build-warning policy is not a replacement for rustdoc lint enforcement.
• No current source, benchmark, or support script was found parsing legacy Rust symbol names.
• The Rust 1.97 upgrade passed Linux, macOS, and Windows CI, but the symbol-mangling and linker_messages conclusions were not recorded explicitly.

Proposed work

• Evaluate Cargo 1.97's build.warnings = "deny" configuration or CARGO_BUILD_WARNINGS=deny for benchmark/build validation currently using RUSTFLAGS='-D warnings'.
  • Preserve strict warnings for local crate targets.
  • Preserve the separate RUSTDOCFLAGS='-D warnings' documentation contract.
  • Confirm that any replacement improves cache reuse and does not weaken feature or benchmark-target coverage.
• Confirm that default v0 symbol mangling requires no repository change because no tooling depends on legacy symbol names. If profiling or symbol-processing assumptions are discovered, update them and add focused validation.
• Inspect supported-platform output for the new linker_messages lint. Fix real linker diagnostics; allow only demonstrated false positives and scope any suppression narrowly.
• Record whether Cargo's resolver.lockfile-path provides a concrete benefit to release benchmarking, historical checkout, or read-only-source workflows. Prefer an explicit no-change conclusion over speculative configuration.
• Record the already-observed no-change conclusions for Result<_, Infallible> must-use behavior and RepeatN::default, which do not materially intersect the current crate.

Acceptance criteria

• Applicable RUSTFLAGS='-D warnings' build/benchmark usage is replaced with Cargo 1.97 warning policy, or retained with a concrete reason.
• Rustdoc warning enforcement remains intact.
• Benchmark compilation remains strict across the normal and exact feature surfaces.
• Symbol-mangling impact is documented as no-change or addressed with focused validation.
• Linux, macOS, and Windows linker-message behavior is reviewed; real warnings are fixed and any suppression is narrowly justified.
• resolver.lockfile-path, Infallible must-use behavior, and RepeatN::default receive explicit adopt/no-change conclusions.
• Repository validation passes without changing numerical results, published APIs, or the Rust 1.97.0 MSRV.

References

• Rust 1.97 upgrade and bit-helper audit: Adopt Rust 1.97.0 and audit exact-arithmetic bit helpers #174
• Rust 1.97 implementation: PR build(msrv)!: require Rust 1.97.0 #183
• Rust 1.97 announcement
• Cargo 1.97 changelog