Support creating assets from PLY files larger than 2GB

[comoc]

Problem

Creating a GaussianSplatAsset from a large PLY file (2.38 GB, ~10.3M splats, full SH) failed — first with a files larger than 2GB are not supported error, and after lifting that check, with a hard Editor crash inside memcpy.

Both issues come from size calculations done in int that overflow once a per-splat buffer exceeds 2GB (InputSplatData is a fixed 248 bytes/splat, so 8.6M splats ≈ 2GB).

Fix

• PLYFileReader: previously loaded the entire binary body into one NativeArray<byte> (capped at ~2GB) and rejected larger files outright. Now opens the stream positioned at the body via OpenAndReadHeader and reads it in 1M-splat batches straight into the destination InputSplatData array (whose element count still fits in an int, so >2GB total is fine).
• ReorderMorton: duplicated the splat array via the NativeArray copy constructor, whose internal size is length * sizeof computed in int — this overflows past 2GB and crashes inside memcpy. Replaced with an explicit long-sized UnsafeUtility.MemCpy and a pointer-based gather.

Notes / scope

• This enables import and rendering of >8.6M-splat clouds. Editing (Edit/Resize) and Export PLY remain bounded by the existing kMaxSplats = 8.6M limit and are out of scope here.
• Tested manually in the Editor with a 2.38 GB / 10.3M-splat PLY at High quality.

[Copilot]

Pull request overview

This PR enables importing very large (>2GB) binary little-endian PLY Gaussian splat files by avoiding int-overflowing size calculations and by streaming PLY body data in batches instead of loading it all at once.

Changes:

• Updated PLYFileReader to expose OpenAndReadHeader(...) so callers can stream the binary body starting from the correct file position (removing the previous “>2GB not supported” behavior).
• Updated GaussianFileReader to read PLY vertex data in 1M-splat batches into a reusable buffer and convert directly into the destination NativeArray<InputSplatData>.
• Updated GaussianSplatAssetCreator.ReorderMorton to avoid NativeArray copy paths that compute byte sizes in int, replacing them with an explicit UnsafeUtility.MemCpy using long.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
package/Editor/Utils/PLYFileReader.cs	Adds a streaming-friendly header reader that returns a positioned FileStream; removes whole-file body loading.
package/Editor/Utils/GaussianFileReader.cs	Streams PLY body data in batches into the destination splat array to support files exceeding ~2GB.
package/Editor/GaussianSplatAssetCreator.cs	Fixes >2GB splat-array reordering by replacing overflow-prone copy logic with long-sized MemCpy and pointer-based gather.

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