Fix CRAN macOS installation and release 0.1.13

[mrchypark]

CRAN macOS builders install Rust under ~/.cargo/bin without adding it to PATH. Although configure finds and validates the toolchain, the build failed on a redundant rustc --version before the existing PATH fallback ran.

Remove that redundant check on Unix and Windows, and use the same Cargo PATH fallback on Windows. Add a macOS CI regression check that removes Rust from PATH. Bump the R package to 0.1.13 and document the CRAN fix.

Validation:

• Reproduced the original configure success followed by rustc: command not found with Rust removed from PATH.
• Local macOS arm64, R 4.5.2: vendored offline R CMD check --as-cran --no-manual with Rust absent from PATH: 0 errors, 0 warnings, 1 note (network-disabled current-time verification). Package loading, examples, and tests passed; the official Debian checkbashisms utility was supplied.
• Official CRAN macOS builder, R 4.6.1 Patched: 0 errors, 0 warnings, 0 notes, including the PDF manual. Results: https://mac.R-project.org/macbuilder/results/1790503825-1beb28f3b17634fc/
• The release archive's build inputs match the committed source. All 11 CI checks passed, including Rust 1.88/stable, macOS with Rust absent from PATH, Windows, Linux x64/arm64, and no-runtime offline checks on R 4.0/release/devel.