docs: verify the README example and finalize the 0.1.0 changelog

[P4suta]

Release preparation for the first windows-spawn publication. conpty-oxide
0.1.2 depends on this crate through a path override and cannot be published
until windows-spawn 0.1.0 is on crates.io.

What this changes

The README example was never compiled. src/lib.rs renders crate
documentation from docs/crate.md, so the README shipped to crates.io without
passing through rustdoc. Its example was written as a doctest (? plus a
trailing Ok::<(), std::io::Error>(())) but nothing type-checked it. Including
the README under cfg(doctest) compiles and runs it without adding a second
front page to the rendered docs. no_run is dropped so it actually executes.

Installation instructions. Replaced the pre-publication note with
cargo add windows-spawn, and stated that the project is unaffiliated with
Microsoft. The windows- prefix collides with the windows-rs crate family
and describes the target platform, not the publisher.

Changelog. Unreleased is folded into 0.1.0. That version was never
tagged or published, so entries filed under Changed and Fixed had no
released baseline to differ from. The reverse inheritance race and the deferred
sha2 bump are recorded as known limitations instead of living only in ADRs.

Verification

just ci        # fmt, clippy, test, doc, msrv 1.75, 3 targets + linux,
               # cargo-deny, reuse, typos, public-api, cargo package + verify
just coverage  # lines 97.30%, regions 94.84%, functions 98.09% (gate 92%)

cargo test --doc --all-features reports 4 doctests, and
src/lib.rs - readme_examples is ok rather than ignored, confirming the
README example executes. cargo public-api is unchanged: this is docs only.

🤖 Generated with Claude Code