docs: update README and CONTRIBUTING to reflect Node.js 25.8.0, tzdata 2026a-r0, and TZ integrity test

[Amnoor]

Summary

This PR updates README.md and CONTRIBUTING.md to bring both documents into alignment with the three develop cycles that have landed since v2.1.0-25.7.0: the Node.js bump to 25.8.0, the tzdata pin bump to 2026a-r0, and the addition of the TZ=UTC integrity test to the PR test suite. Both files also receive a <div align="center"> wrapper around their opening title and icon block to center the heading in GitHub's rendered Markdown view.

Files Changed

Added:

• None

Modified:

• CONTRIBUTING.md
• README.md

Deleted:

• None

Key Changes

• Updated the Node.js Version entry in the README.md Key Features table from 25.7.0 to 25.8.0, and updated the builder stage reference in both the Usage section Dockerfile snippet and the Pinning to a Specific Version example from node:25.7.0-alpine3.22 to node:25.8.0-alpine3.22.
• Updated the Timezone Database row in the README.md Key Features table to specify the explicit package version, changing IANA tzdata included to IANA tzdata 2026a-r0 included.
• Updated all version tag examples in README.md (Usage Dockerfile snippet, Pinning section, and Versioning section) from v2.1.0-25.7.0 to v2.2.0-25.8.0.
• Added a new item 7, Integrity Test: TZ, to the CI/CD & Quality Guarantees numbered list in README.md, documenting that TZ is tested to equal UTC on every pull request.
• Added <div align="center"> / </div> wrappers around the opening title, tagline, and icon block in both README.md and CONTRIBUTING.md, centering the header block in GitHub's Markdown renderer.
• Updated the commit example and PR title example in CONTRIBUTING.md from 25.7.0 to 25.8.0.
• Added the Integrity Test — TZ is UTC description to the PR Tests section in CONTRIBUTING.md immediately after the existing NODE_ENV description, and updated the merge-readiness statement from "All four integrity tests" to "All five integrity tests".
• Updated the Timezone Database row in the CONTRIBUTING.md image components table to specify the package version as IANA timezone data 2026a-r0 rather than the unversioned description.
• Updated the Release Process version tag pattern examples in CONTRIBUTING.md from v2.1.0-25.7.0 / v2.2.0-25.8.0 to v2.2.0-25.8.0 / v2.3.0-25.9.0, and the step 4 tag example from v2.1.0-25.7.0 to v2.2.0-25.8.0.
• Updated the Dockerfile Constraints version example in CONTRIBUTING.md from node:25.7.0-alpine3.22 to node:25.8.0-alpine3.22, and tightened the rejection criteria wording to reference ENV NODE_ENV=production and ENV TZ=UTC with their full ENV prefix rather than just the variable names.