…call/js

Browser WebGPU backend in internal/browser/ (727 LOC, 8 files):
- Instance: navigator.gpu access, RequestAdapter
- Adapter: GPUAdapter wrapper, RequestDevice
- Device: 11 pre-bound creation methods (Ebiten pattern)
- Queue: ref holder (methods in Phase 3)
- Promise utility: goroutine+channel async→sync
- Type conversion: Features (11 mapped), Limits (28 fields)
- Error types: JSError, sentinel errors

Root _browser.go files delegate to internal/browser/.
Zero external dependencies — only syscall/js.
GOOS=js GOARCH=wasm go build . passes.

…sion

internal/browser/: Buffer, Texture, TextureView, Sampler, ShaderModule,
BindGroupLayout, BindGroup, PipelineLayout, RenderPipeline, ComputePipeline.

convert_enums.go: 97 TextureFormats, 31 VertexFormats, all WebGPU enums
(BlendFactor, CompareFunction, StencilOp, AddressMode, FilterMode, etc.)

convert_resources.go: JS descriptor builders for all resource types
including full RenderPipeline descriptor (vertex/fragment/primitive/
depth-stencil/multisample state).

22 table-driven tests cover every enum mapping. 0 lint issues.
Device pre-bound methods wired to create all resource types.
2605 LOC added, zero external dependencies.

CommandEncoder (8 pre-bound methods), RenderPassEncoder (13 methods),
ComputePassEncoder (5 methods), Queue Submit/WriteBuffer/WriteTexture.

Verified against Rust wgpu backend/webgpu.rs patterns:
- Dynamic offsets via Uint32Array (Rust set_bind_group variant)
- WriteBuffer via js.CopyBytesToJS (Rust Uint8Array::from)
- SetVertexBuffer size=0 omits param (Rust no-size variant)
- RenderPass clear value only when loadOp=clear
- TransitionTextures/Buffers no-op (browser handles barriers)
- Command buffers consumed on Submit (marked released)

975 LOC added, zero external dependencies.

Surface wraps HTMLCanvasElement + GPUCanvasContext. Two constructors:
CreateSurface (DOM lookup by data-raw-handle attr or first canvas)
and CreateSurfaceFromCanvas (direct js.Value).

Configure sets canvas dimensions + GPUCanvasContext.configure().
GetCurrentTexture returns frame texture. Present is no-op (browser
auto-presents). GetSurfaceCapabilities returns preferred format via
navigator.gpu.getPreferredCanvasFormat().

Verified against Rust wgpu WebSurface patterns: present=empty,
canvas dimensions in configure(), Mailbox/Immediate rejected.
TextureFormatFromJS reverse map for preferred format detection.

598 LOC added, 29 tests pass, zero external dependencies.

MapAsync/Map with Promise-based goroutine+channel pattern.
MappedRange with lazy copy (Rust OnceCell pattern): Bytes() reads
from JS ArrayBuffer on first access, BytesMut() marks dirty,
Release() auto-flushes dirty data back to JS (Rust Drop pattern).

Verified against Rust wgpu WebBuffer/WebBufferMappedRange:
- getMappedRange called once, ArrayBuffer cached (Rust set_mapped_range)
- Uint8Array sub-range views for offset access
- CopyBytesToGo/CopyBytesToJS for cross-boundary transfer
- mappedAtCreation sets mapState + records range

471 LOC added. All 5 phases complete. Zero external dependencies.

CopyBufferToTexture was pre-bound in internal/browser but missing from
root encoder_browser.go. ClearBuffer also added. Both verified against
Rust wgpu backend/webgpu.rs copy_buffer_to_texture / clear_buffer.

CHANGELOG: browser WebGPU backend (WASM-001) — 5 phases, ~6500 LOC.
README: Browser backend in features table, WASM build instructions.
ROADMAP: v0.28.0, WebAssembly marked DONE, released versions updated.
ARCHITECTURE: dual-path diagram (native/browser), internal/browser/ section.
COMPUTE-BACKENDS: Browser column in support matrix + browser section.

Closes #70 (WASM support request).