Implement native Rust plugin loading

[BSteffaniak]

Add native Rust dynamic library plugin support so Bcode can discover, load, activate, and invoke plugins through versioned interfaces.

Acceptance criteria:

• Plugin manifests can be discovered from supported plugin locations.
• Native dynamic libraries can be loaded from plugin manifests.
• Plugins can expose lifecycle hooks.
• Plugins can register services.
• Plugins can subscribe to host events.
• Bundled plugins are enabled by default.
• Bundled plugins can be disabled through config.
• Plugin load failures are surfaced with actionable diagnostics.