Harden process safety proofs and CI coverage

[passcod]

🤖 This PR hardens the crate’s documented unsafe boundaries and adds CI enforcement for the proofs that can be checked mechanically.

Summary

• add precise local SAFETY: rationales for Unix, Windows, PTY, descriptor-transfer, child-I/O, and test-fixture unsafe operations
• document complete contracts for the public unsafe pre_exec APIs
• enforce unsafe_op_in_unsafe_fn, unused_unsafe, missing_safety_doc, and undocumented_unsafe_blocks
• run Clippy across native frontend/all-feature builds, Rust 1.87.0, and portable PTY targets including build-std coverage for DragonFly and OpenBSD
• add selective whole-target Miri coverage for process-free library and integration-test paths
• record the disposition of the audit findings and draft separate approaches for deferred API and runtime hardening

This PR intentionally makes no runtime behavior, public API signature, private safety-signature, or wrapper-semantics changes. The Miri jobs deliberately exclude native process creation and FFI execution; they exercise only process-free paths that Miri can model.

🤖 Generated with Claude Code