fix(core): scope texture/buffer usage conflicts per pass, not per encoder

[kolkov]

Problem

A render pass followed by a copy of its target in the same encoder fails at Finish:

finish encoder: cannot finish: encoder in Error state: wgpu: texture usage conflict:
track: texture 0 usage conflict: existing 32 incompatible with 4

This is the readback pattern of examples/triangle-headless, which fails like this on current main on Vulkan, DX12 and GLES. Uploading into a texture or vertex buffer and then rendering with it in the same encoder fails the same way.

It is a regression from v0.31.6 (#312): copy usages were merged into one encoder-wide usage scope, and conflicts were checked across the whole encoder. WebGPU defines usage conflicts only inside one usage scope: a render pass, a dispatch, or a single copy command. Between scopes, a resource may change usage freely; the implementation inserts the barrier.

Fix

Usage tracking now follows the Rust wgpu-core tracker model (wgpu-core/src/track/mod.rs):

• Per-scope conflict checks. Each render pass and each copy command gets its own TextureUsageScope / BufferUsageScope. Conflicts are validated only inside it: the same texture as color and depth/stencil attachment, a texture or buffer copied onto itself.
• Per-encoder tracker. New EncoderTextureTracker / EncoderBufferTracker (core/track/encoder_tracker.go) keep the first (start) and last (end) usage of every resource in the command buffer and return the transitions between scopes. This matches TextureTracker::set_single / set_from_usage_scope in track/texture.rs and BufferTracker::set_single in track/buffer.rs. Copy commands record their transitions right before the copy, as command/transfer.rs does (set_single(src, COPY_SRC), set_single(dst, COPY_DST)). Render pass attachments record theirs before the HAL pass begins. Example: ColorTarget → CopySrc before the readback copy.
• Submit-time merge. Barriers go from the device state into the command buffer's start usage, and the device then adopts its end usage. Before this change, one OR-ed usage was used for both.
• CommandEncoder.TransitionTextures updates the tracker, so the next command does not repeat an explicit barrier. This replaces the explicitTextureTransitions special case.

Tests

• New encoder_usage_scope_test.go: render then readback, upload then render, upload then sample, copy then vertex buffer, render → copy → render, all in one encoder. A behaviour-only copy of this file was run against main: 4 of 5 fail with the false conflicts. The fifth (sampled in a pass) passes on main only because bind-group textures are not tracked yet (see below).
• New core/track/encoder_tracker_test.go for start/end/transition bookkeeping.
• Tests that asserted cross-command conflicts now assert the transitions. Self-copies are still rejected atomically.
• examples/triangle-headless: SUCCESS on Vulkan (Khronos validation layer, log file empty at error,warn), DX12 and GLES (Intel Iris Xe). Main fails on all three with the error above.
• gofmt, go build, go test ./..., go mod tidy, golangci-lint on windows/linux/darwin.

Not in this PR (pre-existing gaps, not regressions)

• Bind-group and compute-pass resources are not tracked yet.
• Device tracker never forgets resources (related: core: Texture.Destroy doesn't clear the device-tracker slot before recycling the index #365).
• A copy within one texture is still rejected as a self-copy. Rust allows it between different mip levels or non-overlapping array layers (validate_copy_within_same_texture in command/transfer.rs).
• Vulkan usage → layout mapping for depth and offscreen targets needs a separate review.

The diff is about 800 non-test lines across core/command.go, core/track/ and encoder_native.go. It is one change (the tracker model), so it is not split.

[codecov]

Codecov Report

❌ Patch coverage is 90.51988% with 31 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
core/command.go	75.00%	9 Missing and 10 partials ⚠️
encoder_native.go	92.50%	4 Missing and 2 partials ⚠️
core/track/buffer.go	90.00%	1 Missing and 1 partial ⚠️
core/track/encoder_tracker.go	98.40%	1 Missing and 1 partial ⚠️
core/track/texture.go	90.00%	1 Missing and 1 partial ⚠️

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