feat(plugins): add JavaScript plugin system with overlay panel

- Load plugins from <config dir>/plugins/<id>/ when enabled under `plugins` in config
- Expose ctx with mpv events/commands, timers, subtitle context, actions, notify, exec, and panel
- Bind plugin actions via `__plugin-action:<id>/<action>` keybindings (runPluginAction session action)
- Add plugin-panel modal with markdown rendering; closing aborts the panel signal
- Add ai-explain example plugin that queries `codex exec` for the current line
- Add e2e harness `files` option and plugin e2e scenario; document plugins in docs-site and internal docs

src/renderer/modals/plugin-panel-render.test.ts

@@ -0,0 +1,70 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import { parseInlineMarkdown, parsePluginPanelMarkdown } from './plugin-panel-render';
+
+test('parses the block structure of a typical grammar explanation', () => {
+  const blocks = parsePluginPanelMarkdown(
+    [
+      '## Translation',
+      'I already told you,',
+      "didn't I?",
+      '',
+      '- **言った** past of 言う',
+      '- `じゃない` seeks agreement',
+      '',
+      '1. first',
+      '2. second',
+      '',
+      '> 言ったじゃない',
+      '---',
+      '```',
+      '<b>kept literal</b>',
+      '```',
+    ].join('\n'),
+  );
+
+  assert.deepEqual(
+    blocks.map((block) => block.kind),
+    ['heading', 'paragraph', 'list', 'list', 'quote', 'rule', 'code'],
+  );
+  assert.deepEqual(blocks[1], {
+    kind: 'paragraph',
+    lines: [
+      [{ kind: 'text', value: 'I already told you,' }],
+      [{ kind: 'text', value: "didn't I?" }],
+    ],
+  });
+  assert.deepEqual(blocks[2], {
+    kind: 'list',
+    ordered: false,
+    items: [
+      [
+        { kind: 'strong', value: '言った' },
+        { kind: 'text', value: ' past of 言う' },
+      ],
+      [
+        { kind: 'code', value: 'じゃない' },
+        { kind: 'text', value: ' seeks agreement' },
+      ],
+    ],
+  });
+  assert.deepEqual(blocks[6], { kind: 'code', value: '<b>kept literal</b>' });
+});
+
+test('inline parsing keeps HTML and unmatched markers as text', () => {
+  assert.deepEqual(parseInlineMarkdown('<img src=x> and *em* and 2 * 3'), [
+    { kind: 'text', value: '<img src=x> and ' },
+    { kind: 'em', value: 'em' },
+    { kind: 'text', value: ' and 2 * 3' },
+  ]);
+});
+
+test('underscores inside words stay literal while delimited underscore emphasis still works', () => {
+  assert.deepEqual(parseInlineMarkdown('set ctx_settings_value and _note_ or __bold__'), [
+    { kind: 'text', value: 'set ctx_settings_value and ' },
+    { kind: 'em', value: 'note' },
+    { kind: 'text', value: ' or ' },
+    { kind: 'strong', value: 'bold' },
+  ]);
+});