Add asset load event

[slimbuck]

This PR adds an event fire on PLY asset load.

The event is called load:data and sends the newly created GSplatData instance so the user can modify it before constructing the GSplatInstance.

This was needed so we can continue to support things like 2dgs in supersplat (playcanvas/supersplat#528).

[Copilot]

Pull Request Overview

This PR introduces a new load:data event on PLY asset load so consumers can intercept and modify the parsed GSplatData before it’s turned into an instance.

• Fires load:data on the asset with the raw data object after parsing.
• Ensures reordering step still occurs after event handlers run.
• Enables external extensions (e.g., 2dgs support) to hook into PLY processing.

Comments suppressed due to low confidence (3)

src/framework/parsers/ply.js:613

• Add a unit or integration test to verify that the load:data event is emitted and that event handlers can observe and modify the data object as expected.

                asset.fire('load:data', data);

src/framework/parsers/ply.js:612

• [nitpick] Consider updating the public API docs or adding a JSDoc comment here to explain when and why consumers should listen for load:data, and what shape of data they can expect.

                // allow application to process the data

src/framework/parsers/ply.js:613

• [nitpick] Review existing asset event naming conventions (e.g., use of colons vs. hyphens) to ensure load:data aligns consistently with other events in the framework.

                asset.fire('load:data', data);