ui: keep the board visible in light mode + bust diagram cache

Light mode: revert the heavy white-out scrim back to a moderate wash so the
board stays visible, and earn legibility with a soft light text-halo behind
running text instead (invisible on the solid cards, only works over the image).
Pairs with the darker light-mode text tokens.

Also append ?v=2 to every /diagrams/*.svg reference so the recoloured diagrams
bypass Cloudflare's 4h edge cache and show immediately (the SVG URLs aren't
content-hashed like the CSS bundle).

src/styles/global.css

@@ -409,23 +409,33 @@ svg {
     );
 }
 [data-theme="light"] .site-bg__scrim {
-  /* Light mode needs a much heavier wash than dark — the sandstone board has
-     bright AND mid-tone areas that wreck text contrast. Keep it a faint ghost
-     behind content (dark mode stays the showcase). */
+  /* Moderate wash — the board stays clearly visible; text legibility comes from
+     a light halo behind the type (see below), not from hiding the backdrop. */
   background:
     linear-gradient(
       90deg,
-      rgba(246, 248, 251, 0.96) 0%,
-      rgba(246, 248, 251, 0.9) 50%,
-      rgba(246, 248, 251, 0.86) 100%
+      rgba(246, 248, 251, 0.8) 0%,
+      rgba(246, 248, 251, 0.54) 48%,
+      rgba(246, 248, 251, 0.44) 100%
     ),
     linear-gradient(
       180deg,
-      rgba(246, 248, 251, 0.5) 0%,
-      rgba(246, 248, 251, 0.66) 55%,
-      rgba(246, 248, 251, 0.9) 100%
+      rgba(246, 248, 251, 0.28) 0%,
+      rgba(246, 248, 251, 0.48) 55%,
+      rgba(246, 248, 251, 0.78) 100%
     );
 }
+/* Light-mode legibility: a soft light halo behind running text lifts it off the
+   busy board WITHOUT washing the board out. Subtle enough to be invisible on the
+   solid card surfaces; only earns its keep over the image. Dark mode needs none
+   (light text on a dark board already has the contrast). */
+[data-theme="light"]
+  main
+  :where(h1, h2, h3, h4, p, li, dt, dd, figcaption, .eyebrow) {
+  text-shadow:
+    0 0 4px rgba(246, 248, 251, 0.92),
+    0 1px 2px rgba(246, 248, 251, 0.75);
+}
 
 @media (prefers-reduced-motion: reduce) {
   .scroll-progress {