Colour e2e: baseline commit overridable (COLOR_BASE_COMMIT)

On the release branch mobile-parity's <=767px head rules land before the
colour merge, so 'identical to ab7a100' cannot hold; run with
COLOR_BASE_COMMIT=<commit before the colour merge> there. Measured: vs ab7a100
the only difference is those two documented rules.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

craft/scripts/e2e-color-system.mjs

@@ -25,7 +25,10 @@ import { buildProject } from './e2e-color-system-fixture.mjs';
 
 const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
 const BACKEND_DIR = process.env.WHP_BACKEND_DIR || '/workspace/whp-wt-color-system';
-const BASE_COMMIT = 'ab7a100';
+// Pre-colour-feature baseline. On a branch that also carries other head changes (e.g. the release
+// branch, where mobile-parity's <=767px stacking rules land first) point this at the commit just
+// before the colour merge: COLOR_BASE_COMMIT=<sha>.
+const BASE_COMMIT = process.env.COLOR_BASE_COMMIT || 'ab7a100';
 const SHOTS = process.env.COLOR_SHOTS || '';
 if (SHOTS) fs.mkdirSync(SHOTS, { recursive: true });