Fix Rust 1.100 Clippy diagnostics on main - #9822

Summary

Fix the Rust 1.100 Clippy failures on main that also block unrelated PRs (including #9816).

• Replace map(...).unwrap_or_default() with map_or_default (supported by the project's Rust 1.99 MSRV).
• Remove redundant must_use attributes on ExitCode functions and an unnecessary builtin-int clone.
• Preserve the set iterable's lifetime with a narrowly documented clone-lint allowance; retain both native and no-host ctypes branches.
• Scope paste to Windows and runtime jiff to non-Unix targets, matching their actual uses. The build-time jiff dependency is unchanged.

No Unicode casing changes or CI/test skips are included.

Validation

• Passed: CI most-features workspace Clippy with -Dwarnings.
• Passed: C-API Clippy separately with -Dwarnings.
• Passed: repository hooks and formatting; Rust 1.99 Option/Result::map_or_default compilation probe.
• In progress: full workspace/C-API Rust tests and no-host WASM compilation.
• Windows/macOS builds are left to CI; dependency conditions were statically cross-checked.

The independent main Unicode checksum regression is not fixed by this lint-only PR.

AI assistance: OpenAI Codex inspected the diagnostics, implemented the cleanup, and ran verification. An independent Codex review caught and corrected the Windows jiff dependency condition before publication.