feat: add Do / bind / let for Result and ResultAsync

[iwasa-kosui]

Summary

• Add Do + bind + let for both Result and ResultAsync
• bind accumulates values from Result-returning functions, let from pure functions
• Error types accumulate as union, consistent with andThen
• ResultAsync.bind accepts both sync Result and ResultAsync

Closes #679

Usage

Result.Do()
  .bind("user", () => getUser(id))
  .bind("role", ({ user }) => getRole(user))
  .let("label", ({ user, role }) => `${user.name} (${role.name})`)
  .map(({ user, role, label }) => /* ... */)

Test plan

• Runtime tests for Result.Do / bind / let (value accumulation, short-circuit, error propagation, chaining with map/andThen)
• Runtime tests for ResultAsync.Do / bind / let (same cases + sync/async interop)
• Type-level tests verifying accumulated types and error union inference
• All existing tests pass (137 total)

Co-Authored-By: Claude Opus 4.6 (1M context) [email protected]

[changeset-bot]

🦋 Changeset detected

Latest commit: cffd363

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

This PR includes changesets to release 1 package

Name	Type
neverthrow	Minor

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