docs: add taro visual guardrails

scripts/taro-visual-guardrails-test.js

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+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import { spawnSync } from 'node:child_process';
+
+const repoRoot = process.cwd();
+const scriptPath = path.join(repoRoot, 'scripts', 'taro-visual-guardrails.js');
+
+function run(args = []) {
+  return spawnSync(process.execPath, [scriptPath, '--json', ...args], {
+    cwd: repoRoot,
+    encoding: 'utf8',
+    env: {
+      PATH: process.env.PATH || '',
+      Path: process.env.Path || '',
+      SystemRoot: process.env.SystemRoot || '',
+      ComSpec: process.env.ComSpec || '',
+      TEMP: process.env.TEMP || os.tmpdir(),
+      TMP: process.env.TMP || os.tmpdir(),
+    },
+  });
+}
+
+function parseJson(result) {
+  return JSON.parse(result.stdout || '{}');
+}
+
+const current = run();
+assert.equal(current.status, 0, current.stderr || current.stdout);
+const currentPayload = parseJson(current);
+assert.equal(currentPayload.summary.fail, 0);
+assert.ok(currentPayload.summary.scannedFiles >= 6, 'current Taro CSS files should be scanned');
+assert.ok(currentPayload.checks.some(item => item.id === 'css.border_radius' && item.status === 'pass'));
+assert.ok(currentPayload.checks.some(item => item.id === 'css.decorative_effects' && item.status === 'pass'));
+
+const fixtureRoot = fs.mkdtempSync(path.join(os.tmpdir(), 'taro-visual-guardrails-'));
+fs.writeFileSync(path.join(fixtureRoot, 'bad.css'), `
+.hero-card {
+  border-radius: 24px;
+  font-size: 6vw;
+  letter-spacing: -0.04em;
+  background: radial-gradient(circle, #fff, #dbeafe);
+  filter: blur(20px);
+}
+.random-shape {
+  border-radius: 999px;
+}
+`, 'utf8');
+
+const bad = run(['--root', fixtureRoot]);
+fs.rmSync(fixtureRoot, { recursive: true, force: true });
+assert.notEqual(bad.status, 0, 'fixture with visual violations should fail');
+const badPayload = parseJson(bad);
+assert.ok(badPayload.checks.some(item => item.id === 'css.border_radius' && item.status === 'fail'));
+assert.ok(badPayload.checks.some(item => item.id === 'css.viewport_font_size' && item.status === 'fail'));
+assert.ok(badPayload.checks.some(item => item.id === 'css.letter_spacing' && item.status === 'fail'));
+assert.ok(badPayload.checks.some(item => item.id === 'css.decorative_effects' && item.status === 'fail'));
+assert.ok(badPayload.checks.some(item => item.id === 'css.pill_radius_scope' && item.status === 'fail'));
+
+console.log('[PASS] Taro visual guardrails');