Fix MFR registration and shared plugin state

[Dragoy]

Summary

• pass Adobe's callback registration reserved value 8 without changing the embedded PiPL AE_Reserved_Info value
• expose shared global state to MFR callbacks instead of manufacturing concurrent mutable references
• remove the redundant sequence reference from PluginState, which could alias the mutable instance passed to instance callbacks
• publish parameter metadata safely and require the complete shared GlobalData to be Sync
• require MFR sequence state to be Send + Sync and update all threaded examples

Migration note

For effects that set SupportsThreadedRendering, AdobePluginGlobal::handle_command now receives &self. Mutable global fields must use interior synchronization. PluginState no longer contains sequence; instance callbacks already receive the sequence instance as self.

Verification

• cargo check --workspace
• cargo test --workspace
• regression test separating PluginDataEntryFunction2 reserved metadata from the PiPL property
• downstream Tracery MFR build and all-feature checks

The callback value and PiPL property are separate Adobe SDK fields: the callback uses AE_RESERVED_INFO = 8, while AE_Reserved_Info in the PiPL may remain 0.

[Dragoy]

@AdrianEddy