fix(core): require explicit mock adapter construction

[besmpl]

Summary

Remove only the implicit fabricated mock fallback:

• core.NewInstance now remains empty when no registered provider yields an adapter
• RequestAdapter reports that failure instead of returning a fabricated mock adapter
• deterministic tests opt in with NewInstanceWithMock
• existing explicit software/noop registration remains available
• hal/allbackends keeps its current desktop software-backend registration behavior

The public constructors and explicit fallback providers remain available.

Why

A configured provider set that yields no adapter could previously look like successful GPU initialization because core fabricated a test adapter. That masks loader, registration, and platform-support failures.

Failing closed makes provider availability observable while preserving explicit software and mock use.

Validation

Go 1.25.12:

• go test ./core ./hal/allbackends .

Go 1.26.5:

• go test -race ./core .
• go vet ./core ./hal/allbackends .

Focused tests cover empty-provider failure and explicit mock construction.

This correction is independent of the Android backend preview.

Android integration relationship

This Android-independent change is one of five prerequisites replayed in #268. Its canonical candidate head is e65bec3c74c56cd07265c055ce6a7ec7fbedc680; the corresponding replay in #268 ends at a093f8e969f99f55dfcffdb4b8bcba61cc13299a. The #268 description records the current exact integration head and full replay map; it drops this replay after this PR merges. Rust-v29 typed surface parity remains separate in #273. Android runtime and release claims remain in #268.

[besmpl]

Stacked consumer preview: #268 at exact head b11d2671e80081a191705c41c4e77e8b61472017 incorporates this PR's head e97e4901ee76d9e5f587569c073b38114221c4e6 (matching stable patch IDs).

The preview remains explicitly draft/non-mergeable and will be rebased to Android-only commits after this prerequisite lands.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[kolkov]

Eliminating the implicit mock fabrication is the right call — matches Rust wgpu (no mock/noop fallback in Instance::new) and makes registration failures observable rather than silently masked.

Code is correct, tests are thorough, CI clean on all platforms.

Should fix before merge

hal/allbackends/doc.go is stale after the second commit.

The first commit deleted register.go and updated doc.go to say "The software and no-op providers are not registered by this package." The second commit restored register.go (re-adding _ "github.com/gogpu/wgpu/hal/software") but did not update doc.go. Currently the doc claims software is not registered, but register.go imports it.

Suggested: restore the software backend to the registered list in doc.go and keep only noop as "not registered by this package."

Minor (follow-up OK)

After this PR, useMock is true only via NewInstanceWithMock, which never populates deferredGLES. So the mock-removal block in enumerateDeferredGLES (lines 407-409), hasRealAdaptersLocked, and removeMockAdaptersLocked become unreachable dead code. Not blocking — worth a cleanup pass later.

The new TestNewInstanceDoesNotFabricateAdapterWithoutProvider and TestNewInstanceUsesRegisteredProviderWithoutEnablingMock tests cover the key invariants well. LGTM after the doc fix.

[besmpl]

@kolkov Addressed both the requested documentation correction and the optional cleanup in 3e6c836:

• hal/allbackends now accurately documents software as registered and noop as explicit-only;
• removed the now-unreachable deferred-GLES mock removal path and its dead helpers.

The branch is mergeable and all 10 repository Actions checks are green. Could you please take another look when convenient?

[kolkov]

Both items addressed in 3e6c836:

1. doc.go — software correctly listed as registered, noop as explicit-only
2. Dead code removed — hasRealAdaptersLocked, removeMockAdaptersLocked, deferred-GLES mock block (-41 LOC)

CI green.