Resolves #8812

Changes

Adds a friendly error for the case where a value of the wrong dimension is assigned to a p5.strands shared variable. Previously, code like:

​js let worldPosX = sharedVec3(); worldPosX = inputs.position.x; // scalar assigned to vec3 ​

would emit invalid shader source (vec3 = float) and bubble up as a cryptic GPU shader-compiler message such as ERROR: 0:98: 'assign' : cannot convert from 'highp float' to 'highp 3-component vector of float', with no way for the user to map it back to the JS variable that caused it.

After this PR, the same code throws at IR-construction time with:

​ [p5.strands dimension mismatch]: Cannot assign a value of dimension 1 to `worldPosX`, which expects dimension 3. ​

This follows the four-call-site approach discussed in the plan comment on #8812 (factor out a shared helper, throw early via the existing userError convention).

Implementation

A new dimensionMismatchError(declaredDim, actualDim, varName) helper in src/strands/strands_FES.js is wired into the four sites that produce assignment IR nodes for strands-tracked variables:

1. bridge() in src/strands/strands_node.js — primary path for sharedX = value after the transpiler rewrites the assignment to .bridge(...).
2. bridgeSwizzle() in src/strands/strands_node.js — for swizzle writes like sharedX.xyz = value. Declared dimension comes from swizzlePattern.length.
3. swizzleTrap.set in src/strands/ir_builders.js — the existing partial dimension check was refactored to use the shared helper, preserving the broadcast (dim === 1) and exact-match cases.
4. Object.defineProperty setter in src/strands/strands_api.js — for hook-struct field writes like worldInputs.position = newVec. Declared dimension comes from propertyType.dataType.dimension.

Broadcasting from a scalar (dim === 1) is preserved as before, consistent with non-shared assignment behavior.

Tests

Three tests added in both test/unit/webgl/p5.Shader.js and test/unit/webgpu/p5.Shader.js under the p5.strands suite:

• reports a friendly error when assigning a scalar to a sharedVec3 (bridge)
• reports a friendly error on dimension mismatch via swizzle write (bridgeSwizzle)
• does not error when shared variable assignment dimensions match

Test infrastructure note

Two small adjustments in test/unit/webgl/p5.Shader.js weren't part of the fix itself but were needed for the tests to run cleanly:

• The vi.mock factory for strands_FES was extended to include dimensionMismatchError. The existing mock only stubbed userError and internalError, so the new import broke module loading.
• The afterEach mock-clearing hook in the p5.strands error messages suite was changed to beforeEach. The original only cleaned up after each test, so the suite's first test inherited mockUserError.mock.calls[0] from any prior test in the file that had fired a userError. This was a latent fragility that the new tests happened to expose — beforeEach defends against it generally.

Screenshots of the change

N/A — error-message change only. Before/after messages included above.

PR Checklist

• npm run lint passes
• Inline reference is included / updated (N/A — no public API added)
• Unit tests are included / updated