Add SVG cover art to SX Manifesto as s-expression

Soviet constructivist poster with paper texture filters, grid lines,
aged stain spots, and "(<x>)" symbol in red.

Add missing SVG filter primitive tags to both server (html.py) and
client (sx.js): feTurbulence, feColorMatrix, feBlend,
feComponentTransfer, feFuncR/G/B/A, feDisplacementMap, feComposite,
feFlood, feImage, feMorphology, feSpecularLighting, feDiffuseLighting,
fePointLight, feSpotLight, feDistantLight.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/html.py

@@ -94,6 +94,11 @@ HTML_TAGS = frozenset({
     "g", "defs", "use", "text", "tspan", "clipPath", "mask",
     "linearGradient", "radialGradient", "stop", "filter",
     "feGaussianBlur", "feOffset", "feMerge", "feMergeNode",
+    "feTurbulence", "feColorMatrix", "feBlend",
+    "feComponentTransfer", "feFuncR", "feFuncG", "feFuncB", "feFuncA",
+    "feDisplacementMap", "feComposite", "feFlood", "feImage",
+    "feMorphology", "feSpecularLighting", "feDiffuseLighting",
+    "fePointLight", "feSpotLight", "feDistantLight",
     "animate", "animateTransform",
     # Table
     "table", "thead", "tbody", "tfoot", "tr", "th", "td",