Strengthen spawn ownership and align the quality gates

[P4suta]

Completes the in-flight ownership work and closes the gaps that separated this crate from its siblings.

Ownership and ConPTY

• Private running/suspended typestates, unified handle-transfer ownership, and value-based pseudoconsole storage.
• ConPTY startup sets STARTF_USESTDHANDLES with all ordinary standard handles null, matching Microsoft Terminal, so a hosted child can no longer reach the parent's redirected streams. A 32-bit CI leg runs the regression because the failure mode is pointer-width sensitive.
• SuspendedChild::resume requires the primary thread's previous suspend count to be exactly one; external changes are rejected and rolled back.
• Output capture joins both reader threads even when one errors or panics.

Quality gates

• Deny clippy::undocumented_unsafe_blocks. CONTRIBUTING already required a specific safety justification on every unsafe block, but nothing enforced it and five blocks had none.
  • multiple_unsafe_ops_per_block is deliberately not enabled: several blocks adopt two OS resources together to stay exception-safe (see pipe), and splitting them would widen the window in which one handle can leak. The reason is recorded in Cargo.toml.
• Add a compiled and executed Command example. The crate previously had only compile_fail boundary pins, so no usage example was ever type-checked, and the README example had never been compiled at all.
• Add _typos.toml so the spell-check gate has a checked-in configuration like the sibling repositories.

Dependency handling

The release checksum helper no longer relies on LowerHex being implemented for the digest output, so it builds against sha2 0.10 and 0.11 alike.

The bump itself stays deferred, and the reason is now recorded in dependabot.yml: sha2 0.11 requires Rust 1.85, while this crate targets 1.75. Both just msrv and the Rust 1.75 test legs cover the whole workspace, so taking the bump breaks CI even though xtask never ships. That is the real reason #8 fails — the LowerHex compile error was only the first of several failures. #8 should be closed rather than merged.

Verification

Run locally on Windows: fmt, clippy -D warnings, test --workspace --all-targets, test --doc, typos, reuse lint, cargo deny check, and the public-api snapshot check all pass. MSRV and mutation re-audit were not run locally (the toolchain here is mise-pinned); CI covers MSRV.