fix(overlay): gate the maturity legend on known-word highlighting

The session help color legend showed the four tier rows whenever
maturityEnabled was on, even with known-word highlighting off, where no
token is ever tinted. Extract isKnownWordMaturityLegendEnabled so the
legend requires both toggles, matching getKnownWordMaturityEnabled.

src/renderer/modals/session-help.test.ts

@@ -9,7 +9,9 @@ import {
   createSessionHelpModal,
   describeSessionHelpCommand,
   formatSessionHelpKeybinding,
+  isKnownWordMaturityLegendEnabled,
 } from './session-help.js';
+import type { RuntimeOptionId, RuntimeOptionState } from '../../types/runtime-options.js';
 
 test('session help describes sub-seek commands as subtitle-line navigation', () => {
   assert.equal(describeSessionHelpCommand(['sub-seek', 1]), 'Jump to next subtitle');
@@ -104,6 +106,34 @@ test('session help builds rows from canonical session bindings and fixed overlay
   assert.ok(rows.some((row) => row.shortcut === 'Y then D' && row.action === 'Toggle DevTools'));
 });
 
+function booleanRuntimeOption(id: RuntimeOptionId, value: boolean): RuntimeOptionState {
+  return {
+    id,
+    label: id,
+    scope: 'subtitle',
+    valueType: 'boolean',
+    value,
+    allowedValues: [true, false],
+    requiresRestart: false,
+  };
+}
+
+test('maturity legend requires both known-word highlighting and maturity coloring', () => {
+  const highlightOn = booleanRuntimeOption('subtitle.annotation.knownWords.highlightEnabled', true);
+  const highlightOff = booleanRuntimeOption(
+    'subtitle.annotation.knownWords.highlightEnabled',
+    false,
+  );
+  const maturityOn = booleanRuntimeOption('subtitle.annotation.knownWords.maturityEnabled', true);
+  const maturityOff = booleanRuntimeOption('subtitle.annotation.knownWords.maturityEnabled', false);
+
+  assert.equal(isKnownWordMaturityLegendEnabled([highlightOn, maturityOn]), true);
+  assert.equal(isKnownWordMaturityLegendEnabled([highlightOff, maturityOn]), false);
+  assert.equal(isKnownWordMaturityLegendEnabled([highlightOn, maturityOff]), false);
+  assert.equal(isKnownWordMaturityLegendEnabled([maturityOn]), false);
+  assert.equal(isKnownWordMaturityLegendEnabled([]), false);
+});
+
 function colorLegendRows(input: Parameters<typeof buildSessionHelpSections>[0]) {
   const sections = buildSessionHelpSections(input);
   return sections.find((section) => section.title === 'Color legend')?.rows ?? [];

src/renderer/modals/session-help.ts

@@ -1,4 +1,5 @@
 import type { ModalStateReader, RendererContext } from '../context';
+import type { RuntimeOptionId, RuntimeOptionState } from '../../types/runtime-options';
 import {
   buildSessionHelpSections,
   type SessionHelpSection,
@@ -19,6 +20,19 @@ type SessionHelpBindingInfo = {
   fallbackUnavailable: boolean;
 };
 
+/**
+ * Tiers only render when known-word highlighting is also on, matching
+ * getKnownWordMaturityEnabled in anki-integration/known-word-maturity.
+ */
+export function isKnownWordMaturityLegendEnabled(runtimeOptions: RuntimeOptionState[]): boolean {
+  const isOn = (id: RuntimeOptionId): boolean =>
+    runtimeOptions.some((option) => option.id === id && option.value === true);
+  return (
+    isOn('subtitle.annotation.knownWords.highlightEnabled') &&
+    isOn('subtitle.annotation.knownWords.maturityEnabled')
+  );
+}
+
 /**
  * Maturity coloring is a live runtime toggle, so the color legend reads it from
  * runtime options instead of the resolved subtitle style. A missing or failing
@@ -26,11 +40,7 @@ type SessionHelpBindingInfo = {
  */
 async function readKnownWordMaturityEnabled(): Promise<boolean> {
   try {
-    const runtimeOptions = await window.electronAPI.getRuntimeOptions();
-    return runtimeOptions.some(
-      (option) =>
-        option.id === 'subtitle.annotation.knownWords.maturityEnabled' && option.value === true,
-    );
+    return isKnownWordMaturityLegendEnabled(await window.electronAPI.getRuntimeOptions());
   } catch {
     return false;
   }