fix: add writable mapped-range API across backends

[tarmo888]

Summary

Add a common writable mapped-range API to the native wgpu backend:

BytesMut() []byte
Flush() error

The browser and Rust backends already expose equivalent behavior. This change brings the native backend into API parity so higher-level libraries such as g3d can write mapped buffer data without build-tag-specific code.

Motivation

MappedRange.Bytes() is currently suitable for reading, but higher-level code that writes mapped buffer contents needs an explicit writable-range API.

The browser backend uses a Go-side staging slice for mapped writes and requires Flush() to copy modified data back to the JavaScript ArrayBuffer. The Rust backend exposes a direct writable mapping. Native mappings also expose direct writable memory.

Without a common API, consumers need platform-specific implementations for the same geometry or buffer upload code.

Changes

• Added MappedRange.BytesMut() to the native backend.
• Added MappedRange.Flush() to the native backend.
• Implemented native BytesMut() as an alias of Bytes().
• Implemented native Flush() as a no-op because native mappings are directly device-visible.
• Added a regression test

Related

This change is a prerequisite for the corresponding gogpu/g3d update that switches geometry uploads to the common BytesMut() and Flush() API.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[kolkov]

Clean PR — fills the native backend gap for BytesMut()/Flush() that browser and Rust backends already have. CI 16/16 pass.

Implementation is correct: native BytesMut() = alias of Bytes(), Flush() = no-op. This matches the Rust backend exactly (mapped_range_rust.go uses the same pattern).

Test follows existing conventions (newDevice + requireHAL + MappedAtCreation).

Design note (non-blocking, for awareness)

Flush() as no-op is correct for DX12, Metal, GLES, Software, and Noop — all return IsCoherent: true from MapBuffer. Vulkan CAN return IsCoherent: false for non-coherent memory types, where CPU writes aren't GPU-visible without vkFlushMappedMemoryRanges. However, this is a pre-existing gap — MappedRange doesn't store the coherency flag, and Bytes() already allows writes with no flush mechanism. The Vulkan UnmapBuffer handles the flush. A proper Flush() for non-coherent Vulkan would need plumbing IsCoherent into MappedRange — that's a separate enhancement.

LGTM.