feat(decnet_web/theme-lab): light theme tokens + dev toggle

Adds html[data-theme="light"] block to index.css overriding the
core six tokens (bg, matrix, violet, panel, border, alert), the
matrix/violet/alert tints, and the foreground opacity ramp to a
cream-on-ink palette anchored on #dbdad6. Glows are no-op'd —
light mode trades neon haloes for hard 1px borders.

Lab page gets a Dark/Light toggle that flips
html.dataset.theme and persists to sessionStorage
(decnet_theme_lab) — intentionally tab-scoped, not user-facing.
App.tsx hydrates the same key on boot so a tab reload keeps the
dev's chosen theme. The user-facing localStorage toggle ships
later via Config.

decnet_web/src/components/ThemeLab/ThemeToggle.tsx

@@ -0,0 +1,57 @@
+import React, { useEffect, useState } from 'react';
+
+/* Dev-scoped Dark/Light theme toggle for the theme lab.
+ *
+ * Flips `document.documentElement.dataset.theme` and persists to
+ * **sessionStorage** intentionally — the lab is a tab-scoped
+ * exploration tool. Global persistence (across reloads, all users)
+ * is the user-facing Config toggle that ships in Task 6. */
+
+export const THEME_SESSION_KEY = 'decnet_theme_lab';
+
+export type Theme = 'dark' | 'light';
+
+export function readLabTheme(): Theme {
+  try {
+    const v = sessionStorage.getItem(THEME_SESSION_KEY);
+    return v === 'light' ? 'light' : 'dark';
+  } catch {
+    return 'dark';
+  }
+}
+
+export function applyTheme(theme: Theme): void {
+  document.documentElement.dataset.theme = theme;
+}
+
+const ThemeToggle: React.FC = () => {
+  const [theme, setTheme] = useState<Theme>(() => readLabTheme());
+
+  useEffect(() => {
+    applyTheme(theme);
+    try { sessionStorage.setItem(THEME_SESSION_KEY, theme); } catch { /* ignore */ }
+  }, [theme]);
+
+  return (
+    <div className="lab-theme-toggle" role="group" aria-label="Theme">
+      <button
+        type="button"
+        className={`btn small ${theme === 'dark' ? '' : 'ghost'}`}
+        onClick={() => setTheme('dark')}
+        aria-pressed={theme === 'dark'}
+      >
+        DARK
+      </button>
+      <button
+        type="button"
+        className={`btn small ${theme === 'light' ? '' : 'ghost'}`}
+        onClick={() => setTheme('light')}
+        aria-pressed={theme === 'light'}
+      >
+        LIGHT
+      </button>
+    </div>
+  );
+};
+
+export default ThemeToggle;