[Atoms][Experimental] Compile runtime Document classNames into CSS

[MenKNas]

• This PR follows the Contribution Guide
• Changelog updated
• Upgrade guide entry added

Description / Motivation

This PR implements runtime CSS compilation for Atoms Document classNames based on Nick’s POC (#558 / 5b48f34).

Tailwind can’t see classes that only exist in MMS Document JSON at build time, so styles never ship. This compiles those classes at runtime and injects the CSS for Design Library editing and production RSC rendering.

Defaults to Tailwind, with a pluggable compiler registry for custom setups.

What was added extra beyond the POC:

• Fixed Tailwind build() candidate accumulation so only current Document classes are in the injected CSS (not stale ones from earlier updates)
• Wired both App Router starters (instrumentation, compileCssAction, serverExternalPackages)
• Added caching, clearer missing-compiler/CSS-file DX, and unit coverage for the accumulation fix

Testing Details

• Unit Test Added
• Manual Test/Other (Please elaborate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[changeset-bot]

🦋 Changeset detected

Latest commit: 1894db7

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

This PR includes changesets to release 4 packages

Name	Type
@sitecore-content-sdk/core	Minor
@sitecore-content-sdk/react	Minor
@sitecore-content-sdk/nextjs	Minor
create-content-sdk-app	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[art-alexeyenko]

I do like how we just keep finding barely-public APIs for these kinds of things. Let's refine some more and bring it to a close.

[art-alexeyenko]

Approving - let's just add some CSS compilation guides and pointers in Pages Router sample before merge.