feat(result): add unwrapOr, unwrapOrElse, map, and mapFailure

[coderdan]

Summary

Follow-up to #37 (unwrap/expect). Adds four more Rust-inspired combinators to @byteslice/result, all as free functions consistent with the package's existing functional surface:

• unwrapOr(result, fallback) — returns the success data, or an eager fallback value. Never throws.
• unwrapOrElse(result, fn) — returns the success data, or computes a fallback from the failure (fn is only invoked on failure). Never throws.
• map(result, fn) — transforms the success data (Result<S, F> → Result<U, F>), passing a failure through untouched.
• mapFailure(result, fn) — transforms the failure (Result<S, F> → Result<S, G>), passing a success through untouched.

Design notes

• Total extractors don't throw. Unlike unwrap/expect, the unwrapOr* pair always returns a value — fitting TS's preference for not throwing on expected paths. They correctly return falsy success data (e.g. 0, '') rather than the fallback, since they branch on the success/failure discriminant, not truthiness.
• unwrapOrElse and mapFailure receive the full failure (F), not an extracted Error — mirroring Rust's E and giving callers access to custom-failure fields. (unwrap/expect extract an Error only because they throw.)
• map is synchronous and non-catching, matching Rust's infallible closure. If a transform can fail, withResult is the right tool — called out in the docs.
• mapFailure's mapped failure is constrained G extends FailureOption, so you can't map to an invalid failure shape.

Testing

• 16 new unit tests (27 total), covering success/failure paths, falsy-data handling, lazy-callback non-invocation, and custom failures.
• bun test, biome ci, cspell, and tsup build all pass.

Changeset

Included a minor changeset. Also added combinator/combinators to the cspell word list.

[changeset-bot]

🦋 Changeset detected

Latest commit: dc78c5e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@byteslice/result	Minor
@byteslice/events	Patch
@byteslice/playground	Patch

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[electric-gizzard]

@coderdan your "Rust zealot[ry]" is showing 🦀

Thank you for contributing!

[coderdan]

@coderdan your "Rust zealot[ry]" is showing 🦀

Thank you for contributing!

It's a sickness, what can I say! Thanks for the quick approval!