feat: use selected Chatroom brand artwork

[NWYLZW]

Change

Chatroom now exports the selected 256×256 brand artwork through the existing public CordisXPluginModule.icon contract, so Host plugin lists and details use its own mark. The original PNG is tracked at assets/chatroom.png and included in the package allowlist for immutable Marketplace URLs. Navigation/action icons, runtime manifest, version and private local distribution remain unchanged.

Validation

• Selected PNG SHA256: 6cfaaac15f1e7d325f2db248aaf28981c22460797f090ba30fc558a54d711620; dimensions and embedded bytes verified.
• Executed the pinned Host pluginBrandIconDataUrl implementation against the exported icon: accepted.
• Focused regression checks PNG/embedded parity and production graph inclusion; full owner check, source lint and formatting run in CI.
• Local formatting and diff check passed; npm pack dry-run includes the source PNG. Local full dependency installation was stopped to avoid concurrent recursive Git dependency prepares on the shared machine.
• No real native renderer claim: Host consumption is confirmed through the pinned implementation; live app validation remains separate.

Distribution remains repository source / explicit-local-v1, with no npm publish workflow or version bump.