Adopt Rust 1.97.0 and audit exact-arithmetic bit helpers

[acgetchell]

Summary

Raise the la-stack MSRV/toolchain baseline from Rust 1.96 to Rust 1.97.0 once Rust 1.97.0 is available on stable, then deliberately audit the new 1.97.0 bit-operation APIs for exact-arithmetic and benchmark hot paths.

This should stay focused on correctness, invariant clarity, and measured performance. The goal is not to rewrite code just because a new standard-library method exists.

Current State

• Cargo.toml currently declares rust-version = "1.96".
• rust-toolchain.toml currently pins channel = "1.96.0".
• The exact arithmetic implementation in src/exact.rs already does substantial bit-level work:
  • IEEE-754 f64 decomposition with mantissa/exponent extraction.
  • NonZeroU64 mantissa normalization via trailing-zero stripping.
  • exact rational-to-f64 conversion using denominator power-of-two checks.
  • BigInt bit-length and trailing-zero analysis for exact representability and rounding decisions.
• Rust 1.97.0 stabilizes primitive integer and NonZero helpers such as bit_width, highest_one, lowest_one, isolate_highest_one, and isolate_lowest_one.

Proposed Changes

• Bump the Rust baseline to 1.97.0 in repository metadata and documentation.
• Audit src/exact.rs, especially:
  • f64_decompose
  • bigint_exp_to_bigrational
  • exact_rational_to_finite_f64
  • bigint_exp_to_finite_f64
  • exact determinant/sign helpers that operate on primitive mantissas before promotion to BigInt
• Use Rust 1.97.0 bit helpers where they make exactness invariants clearer, remove hand-rolled bit arithmetic, or improve performance.
• Keep num-bigint's bits() and trailing_zeros() APIs where the value is already a BigInt; do not contort code to use primitive helpers after promotion.
• Audit benchmark utilities such as benches/common/exact.rs for small, useful NonZero* or bit-width simplifications.
• Review the remaining Rust 1.97.0 surface (RepeatN::default, char::is_control const-stability, LoongArch feature detection, Cargo 1.97 diagnostics/CLI changes) and explicitly skip it unless it has a concrete la-stack benefit.

Acceptance Criteria

• Cargo.toml, rust-toolchain.toml, README, AGENTS.md, release docs, and CI/tooling docs consistently declare Rust 1.97.0 as the MSRV/toolchain baseline.
• The PR includes a short audit note separating true Rust 1.97.0 features from already-baseline Rust 1.95/1.96 cleanup.
• src/exact.rs is audited for bit_width, highest_one, lowest_one, isolate_highest_one, and isolate_lowest_one; any adoption preserves exact arithmetic semantics and has focused tests.
• Existing exact tests and proptests continue to cover D=2 through D=5 where applicable, including non-finite rejection, exact representability, overflow/unrepresentability, and rounded-vs-exact behavior.
• Exact-arithmetic benchmark evidence is captured before and after relevant changes, using just bench-exact or a focused Criterion filter when the full suite is unnecessary.
• num-bigint bit APIs remain in use where the data is already arbitrary precision; rejected primitive-helper opportunities are briefly documented in the PR notes.
• Cargo 1.97, Clippy, rustdoc, and compatibility-note changes are audited; new diagnostics are fixed or narrowly justified.
• No unsafe Rust is introduced.
• Final validation passes under Rust 1.97.0 stable: just ci, including exact-feature checks.
• Changelog-ready release notes mention the Rust 1.97.0 MSRV bump and any exact-arithmetic bit-helper changes.

Benefits

• Keeps la-stack aligned with the Rust toolchain baseline used by downstream geometry crates.
• Uses the new integer/NonZero helpers where they directly express exact-arithmetic invariants.
• Gives exact-arithmetic performance work a concrete benchmark-before/after requirement.
• Preserves the v0.4.x roadmap focus on stable Rust, correctness, benchmark coverage, and release hardening without pulling in the v0.5.0 generic_const_exprs work.

Implementation Notes

• This belongs in the stable-Rust v0.4.x line, not the v0.5.0 generic_const_exprs milestone.
• Start once Rust 1.97.0 is available on stable.
• Related local code paths: src/exact.rs, benches/exact.rs, and benches/common/exact.rs.