[Task] Add a community-tested platform matrix

[ryan-wong-coder]

Background

TrustDB publishes binaries for several operating systems and architectures, but users cannot currently see which quick-start and offline-verification flows have been community tested.

Scope

Add a lightweight platform matrix covering CLI install, local L3 demo, server start, .sproof export, and offline verification.

Acceptance criteria

• Separates CI-tested, maintainer-tested, and community-reported results.
• Records TrustDB version, OS, architecture, and test date.
• Provides a simple pull-request process for adding a result.
• Does not imply production certification.
• Links to platform-specific known issues where applicable.