chore: update MSRV to Rust 1.95.0

[acgetchell]

Summary

Update the minimum supported Rust version (MSRV) from 1.94 to 1.95.0 and adopt relevant new features. Slated for v0.7.6.

Motivation

• Rust 1.95.0 was released on 2026-04-16
• la-stack (a dependency) is being upgraded to Rust 1.95.0, so delaunay's declared MSRV of 1.94 would become misleading — Cargo's MSRV-aware resolver would require 1.95 anyway through the dependency chain
• Several stabilized features are directly useful for this codebase

New Features to Adopt

Language

• if let guards on match arms (rust-lang/rust#141295) — The codebase has many match arms with nested if let that can be flattened into guard position, improving readability

Stabilized APIs

• assert_matches! / debug_assert_matches! — Now stable; useful in tests for pattern-matching assertions
• core::hint::cold_path — Can annotate error/unlikely branches in hot paths for better code layout
• cfg_select! — Stable replacement for cfg_if!-style conditional compilation patterns

Other

• bool: TryFrom<{integer}> — May simplify some conversions
• LLVM 22 update — potential codegen improvements

Tasks

• Update rust-version in Cargo.toml from "1.94" to "1.95"
• Update rust-toolchain.toml and CI configuration if applicable
• Update MSRV references in AGENTS.md and docs/
• Refactor match arms to use if let guards where it improves clarity
• Replace manual pattern-matching assertions in tests with assert_matches!
• Evaluate hot paths for core::hint::cold_path annotation opportunities
• Run just ci to verify everything passes
• Coordinate with la-stack MSRV bump

[acgetchell]

The assert_matches! / debug_assert_matches! bullet is being deferred and tracked separately in #329.

Stabilization PR rust‑lang/rust#137487 was tagged milestone 1.95.0 but did not actually ship stable in rustc 1.95.0. The shipped library source still carries #[unstable(feature = "assert_matches", issue = "82775")] on the assert_matches macro, while cfg_select! in the same file did ship as #[stable(..., since = "1.95.0")].

All other items in this issue remain in scope for the 1.95 adoption PR.

[acgetchell]

Status update on remaining bullets

core::hint::cold_path — extended coverage

Added cold_path() annotations at three additional sites that match the Stage 1→Stage 2 pattern already established in src/geometry/predicates.rs:

• src/geometry/robust_predicates.rs::robust_insphere — Strategy 3 (insphere_distance + SoS fallback), reached only when the exact-sign dispatcher fails (e.g. D ≥ 6 where the matrix exceeds the stack-matrix limit).
• src/geometry/robust_predicates.rs::adaptive_tolerance_insphere — Orientation::DEGENERATE early-return.
• src/geometry/util/circumsphere.rs::circumcenter — Err(LaError::Singular) branch (solve_exact_f64 BigRational fallback) and the general LU-factorization failure branch.

Other candidates considered and not annotated (rationale):

• Wrong-point-count validation errors in robust_insphere / robust_orientation / circumcenter: bodies are tiny (error construction only), hardware branch predictor handles them; cold_path would add noise without meaningful layout benefit.
• Error/debug branches in flips.rs (repair trace, skip logging): already guarded by repair_trace_enabled()‑style boolean checks that the optimizer can hoist; adding cold_path duplicates that hint.
• locate.rs error paths: construction‑only, same reasoning as the validation errors above.

All cold_path additions compile and pass cargo clippy --lib --all-features -- -D warnings.

bool: TryFrom<{integer}> — evaluated, no candidates

Grepped for as bool, != 0 / == 0 / == 1 used as integer→bool conversions, integer‑keyed match arms returning bool, and u8/u32 flags converted to bool. Found none: the codebase uses strongly‑typed enums (InSphere, Orientation, LocateResult, InsertionOutcome, FlipDirection, etc.) and Option<_> for binary/ternary state rather than integer flags. bool::try_from(integer) would not simplify any call site in src/.

Checklist summary

• rust-version in Cargo.toml bumped to 1.95
• rust-toolchain.toml / clippy.toml bumped to 1.95
• MSRV references in AGENTS.md and CONTRIBUTING.md updated
• if let guards adopted in src/core/algorithms/flips.rs and src/triangulation/delaunayize.rs
• assert_matches! adoption — deferred to chore: adopt assert_matches! in tests once it reaches stable #329 (stabilization PR rust‑lang/rust#137487 did not ship stable in rustc 1.95.0)
• core::hint::cold_path — extended coverage as above
• bool: TryFrom<{integer}> — evaluated; no candidates in this codebase
• Coordinate with la-stack MSRV bump (0.4.0 → 0.4.1)
• just ci full run — pending on PR