Docs: brand public README and correct architecture docs

[silahian]

Summary

• Brand the public README with official light and dark wordmarks and public-core market visuals.
• Add concise community and update links, including Discord, LinkedIn, X, and Substack.
• Correct the public architecture pages to reflect the current plugin, helper, snapshot, and trigger paths.

Validation

• Local Markdown links passed.
• GitHub Markdown rendering passed.
• git diff --check passed.

[Copilot]

Pull request overview

This PR updates the public-facing documentation to better reflect the current VisualHFT branding and the repository’s actual architecture boundaries (desktop host, shared Commons libraries, and plugin lifecycle), while refreshing community/update links.

Changes:

• Branded the root README with light/dark wordmarks, streamlined navigation, and added community/update links.
• Rewrote the architecture overview to match current plugin/helper/snapshot/trigger concepts and added a source map.
• Updated the interactive architecture HTML page and simplified the extending guide.

Reviewed changes

Copilot reviewed 4 out of 6 changed files in this pull request and generated no comments.

File	Description
README.md	Adds branded header, refreshed navigation, and updated community/update links.
docs/system-architecture.html	Updates the interactive architecture narrative to match current shared helpers, snapshot usage, and plugin lifecycle; removes Chart.js content.
docs/extending/README.md	Removes the license-level section and keeps the extension loading guidance focused and concise.
docs/architecture.md	Replaces the older overview with a mermaid “at a glance” diagram, clearer component boundaries, lifecycle notes, and a source map.

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