docs: document --stableTypeOrdering impact on packaging tests

[martyy-code]

Description

Document the impact of TypeScript 6's --stableTypeOrdering flag on packaging tests (publint, attw). When stableTypeOrdering is enabled, union types and object keys are emitted in a deterministic order, which affects declaration emit and can cause attw to fail unexpectedly.

Why

TypeScript 6 introduces --stableTypeOrdering to normalize declaration output. When enabled, union order in .d.ts files becomes predictable. attw resolves types under node16/nodenext/bundler and can flag these as differences. Without documentation, consumers will hit mysterious CI failures.

Acceptance criteria

• Document the stableTypeOrdering flag and its effect on declaration emit
• Document how it interacts with publint and attw
• Provide a workaround or config if order matters

Risks

• None. Pure documentation.

Related

• https://devblogs.microsoft.com/typescript/announcing-typescript-6-0/