style(site): noise texture on buttons and h1/h2 lettering

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

site/includes/include.php

@@ -89,4 +89,20 @@ $canonical = $GLOBALS['_canonical_override'] ?? match ($page) {
 <?php include dirname(__DIR__, 2) . '/includes/env_ribbon.php';   // moldura de aviso fora de produção ?>
 </head>
 <body>
+<svg width="0" height="0" style="position:absolute" aria-hidden="true" focusable="false">
+  <!-- Grain for the h1/h2 lettering (style.css, "Noise texture"): fractal noise -> grey ->
+       squeezed to 72-100% brightness, multiplied into the glyphs and clipped to their shape. -->
+  <filter id="pt-noise" x="0" y="0" width="100%" height="100%" color-interpolation-filters="sRGB">
+    <feTurbulence type="fractalNoise" baseFrequency=".9" numOctaves="2" seed="4" result="n"/>
+    <feColorMatrix in="n" type="saturate" values="0" result="g"/>
+    <feComponentTransfer in="g" result="grain">
+      <feFuncR type="linear" slope=".3" intercept=".72"/>
+      <feFuncG type="linear" slope=".3" intercept=".72"/>
+      <feFuncB type="linear" slope=".3" intercept=".72"/>
+      <feFuncA type="linear" slope="0" intercept="1"/>
+    </feComponentTransfer>
+    <feBlend in="SourceGraphic" in2="grain" mode="multiply" result="mix"/>
+    <feComposite in="mix" in2="SourceGraphic" operator="in"/>
+  </filter>
+</svg>
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