test(shader): variants through the Shader API and through Build()

ShaderCompile registers one source twice and checks that the variant
gets its own output name, takes the other #ifdef branch and receives
the value of a NAME=VALUE define. Staleness is pinned per output: while
only the plain output exists the variant must still be due, or Build()
would never compile a variant added to a built project. Malformed
defines and a variant with a path separator must be refused by name.

ShaderVariantBuild drives the same through Build(), which is where the
entries compile in parallel into one directory: a variant added to an
already built configuration is compiled into its own file, and two
entries with one output name fail the build without touching the
existing output. Its fixture tests the macro with `#if`, which pins
that a bare define is 1 and not empty.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/ShaderVariantBuild/fixture/main.cpp

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+int main() {
+    return 0;
+}