docs: add OR_DO_THIS.md zero-dep cancellation pattern

[lilith]

Summary

• Adds OR_DO_THIS.md — a one-file, zero-dep cooperative cancellation
  pattern for crates that can't or won't take the enough dep. Same
  Option<Arc<dyn Fn>> shape as enough::StopToken, closure-backed so
  any source (AtomicBool, tokio CancellationToken, enough::Stopper,
  channels, TLS) bridges in one line. StopReason matches enough's
  variant names so impl From<StopReason> is portable.
• Adds tests/test-or-do-this/ — a validation crate containing the
  copy-pasteable zerodep.rs file and 32 unit + 6 doctests that lock
  in every claim in the document.
• Naming note: StopCheck deliberately differs from StopToken to
  disambiguate closure-backed vs trait-backed representations when
  both crates are in scope. StopReason deliberately matches so
  error-type impls port without churn.

What's validated

• Standalone use — none(), new(), from_flag(), check()?,
  Clone, Send + Sync, const-ness, core::error::Error.
• Storage without lifetime parameters (no viral Decoder<'a>).
• Forward adapter (enough user calls StopCheck-taking lib) via
  closure, with both from_flag and reason-preserving new paths.
• Reverse adapter (StopCheck holder calls enough-using lib) via
  almost_enough::FnStop, including a reason-preserving manual
  Stop impl.
• Clone preserved through the full chain both directions, with a
  compile-time assertion that FnStop<move-closure capturing StopCheck>
  is Clone.
• Reason preservation for both Cancelled and TimedOut, including
  actual timeout firing via TimeoutExt::with_timeout.

Test plan

• cargo test -p test-or-do-this — 32 unit + 6 doctests, all pass
• cargo clippy -p test-or-do-this --all-targets -- -D warnings — clean
• CI green across all platforms (Linux, macOS Intel, macOS aarch64, Windows, windows-11-arm, i686-unknown-linux-gnu)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.85%. Comparing base (8f1dbc3) to head (f398ddf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #7   +/-   ##
=======================================
  Coverage   97.85%   97.85%           
=======================================
  Files          16       16           
  Lines        2336     2336           
=======================================
  Hits         2286     2286           
  Misses         50       50           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.