fix(utils): harden package for stable release

[dancormier]

Summary

• validate the packed Stacks Utils package through a clean ESM and TypeScript NodeNext consumer
• fix NodeNext declaration resolution and isolate the custom Day.js locale from consumer applications
• document formatting/runtime contracts and add timezone-safe boundary coverage
• run the package smoke test in CI

Jira

• STACKS-890

Validation

• npm run build -w packages/stacks-utils
• npm run test -w packages/stacks-utils — 22 passed
• npm run lint -w packages/stacks-utils
• npm run test:package -w packages/stacks-utils
• npm run build -w packages/stacks-svelte
• npm run lint -w packages/stacks-svelte
• npm run test -w packages/stacks-svelte — 577 browser tests and 26 tooling tests passed
• independent Day.js locale-mutation check
• git diff --check

Release notes

• includes a patch changeset for @stackoverflow/stacks-utils
• this is release-readiness work; stable publication remains part of the global Changesets prerelease exit after the remaining Stacks packages are ready

Accessibility

• no UI or accessibility behavior changed

[changeset-bot]

🦋 Changeset detected

Latest commit: ed32260

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

This PR includes changesets to release 1 package

Name	Type
@stackoverflow/stacks-utils	Patch

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[dancormier]

Note

This PR is part of a larger effort to close out the beta phase of this repository.

---

Automated review

Findings

No actionable findings. I found no meaningful consumer-visible output or API regressions.

The removed Day.js threshold and rounding settings were previously added through updateLocale, but Day.js only reads those settings when passed to dayjs.extend(relativeTime, options). They were therefore already ineffective. Relative-time strings and effective boundary behavior remain unchanged.

The change improves compatibility by:

• Preserving existing formatTime output.
• Preventing mutation of the consumer’s default English Day.js locale.
• Fixing NodeNext declaration resolution without changing exports.
• Retaining existing ESM runtime imports.

Review summary

• Standards findings: 0
• Spec findings: 0
• Observed checks: All GitHub CI checks passed. The utils unit, build, and packed-package tests also completed successfully during verification.
• Residual risk: Importing the package still globally installs Day.js’s relativeTime plugin and registers the stacks-utils locale. Plugin installation predates this PR; locale-name collision is theoretically possible but unlikely.
• Deferred Jira work: Stable publication, dist-tag correction, and Svelte’s stable dependency remain explicitly deferred to the global release process.