feat: add MCOpenNex update presenter

[ciqueira]

What changed

• adds a shared MCOpenNex client for anonymous update and notice checks;
• starts the check asynchronously outside render and exposes snapshots through the OFX Support group;
• adds non-persistent update status plus Check for Updates and Open MCNexus actions;
• opens only allowlisted mcnexus and HTTPS URLs through native platform APIs;
• documents the runtime boundary and Apache-2.0 attribution.

Runtime behavior

Instances in one host process share the SDK client and check interval. OFX UI actions pull immutable snapshots; render never starts network work or mutates the host UI.

Validation

• universal macOS bundle built for arm64 and x86_64;
• staging endpoint embedded through the build-time override;
• native SDK tests and CI passed on macOS and Windows.

The matching MC-OFX build-system PR adds the pinned SDK checkout, compilation, linkage, and legal bundle files.