feat(asset): remove modifiers from the API [SWC-1264]

[castastrophe]

Description

This update removes --mod-asset-* custom property hooks per SWC-1264, see also the RFC for extensible styling. Class selectors and variants remain unchanged; stories were refreshed to match the current API.

• Remove all --mod-asset-* custom property hooks.
• Reduces unnecessary re-mapping of custom properties in favor of using them directly

Breaking change: the --mod-asset-* override layer is removed. Consumers should set --spectrum-asset-* variables directly where customization as needed.

Related issue(s)

• fixes SWC-1264

Author's checklist

• I have read the CONTRIBUTING and PULL_REQUESTS documents.
• I have reviewed the Accessibility Practices for this feature, see: Aria Practices
• I have added automated tests to cover my changes (or confirmed existing coverage is sufficient for styling-only changes).
• I have included a well-written changeset if my change needs to be published.
• I have included updated documentation if my change required it.

Reviewer's checklist

• Includes a GitHub Issue with appropriate flag or Jira ticket number without a link
• Includes thoughtfully written changeset if changes suggested include patch, minor, or major features
• Automated tests cover all use cases and follow best practices for writing
• Validated on all supported browsers
• All VRTs are passing or approved

Validation steps

• No visual or behavioral changes compared to the current spectrum-two baseline.

Regression testing

Validate:

1. The documentation pages for at least two other components are still loading, including:

   • The pages render correctly, are accessible, and are responsive.

2. If components have been modified, VRTs have been run on this branch:

   • VRTs have been run and looked at.
   • Any VRT changes have been accepted (by reviewer and/or PR author), or there are no changes.

[changeset-bot]

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Latest commit: 32de420

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[github-actions]

📚 Branch preview

PR #4257 has been deployed to Azure Blob Storage: https://spectrumcss.z13.web.core.windows.net/pr-4257/index.html.

[github-actions]

File metrics

Summary

Total size: 1.43 MB^*

Package	Size	Minified	Gzipped
asset	1.17 KB	1.10 KB	0.59 KB

asset

Filename	Head	Minified	Gzipped	Compared to base
index.css	1.17 KB	1.10 KB	0.59 KB	🟢 ⬇ 0.86 KB
metadata.json	0.49 KB	-	-	🟢 ⬇ 0.47 KB

* Size is the sum of all main files for packages in the library.
* An ASCII character in UTF-8 is 8 bits or 1 byte.