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

e2e/plugins.e2e.test.ts

@@ -0,0 +1,150 @@
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import path from 'node:path';
+import test, { after } from 'node:test';
+import type { CdpPage } from './harness/cdp';
+import { FIXTURE_CUES } from './harness/fixtures';
+import { OVERLAY_PAGE, startE2eSession, type E2eSession } from './harness/session';
+import { waitUntil } from './harness/wait';
+
+// Bun caps node:test hooks at 5s whatever their timeout option says, so the
+// session boots inside whichever test runs first and every test budgets for it.
+const TEST_TIMEOUT_MS = 120_000;
+
+// Opens a panel describing the subtitle context and a child process result, and
+// logs when the panel's signal aborts so the test can see the close reach main.
+const PROBE_PLUGIN = `
+exports.activate = (ctx) => {
+  ctx.actions.register('show', () => {
+    const { before, current, after } = ctx.subtitles.context({ before: 1, after: 1 });
+    const panel = ctx.ui.panel({ title: 'Probe', markdown: '_working_' });
+    panel.signal.addEventListener('abort', () => ctx.log.info('probe panel aborted'));
+    ctx
+      .exec(process.execPath, ['-e', 'process.stdout.write("child-ok")'], {
+        env: { ELECTRON_RUN_AS_NODE: '1' },
+        signal: panel.signal,
+      })
+      .then((result) =>
+        panel.update({
+          markdown: [
+            '## ' + ctx.settings.greeting,
+            '',
+            '- before: ' + before.map((line) => line.text).join('|'),
+            '- current: **' + (current ? current.text : '') + '**',
+            '- after: ' + after.map((line) => line.text).join('|'),
+            '- exec: ' + result.stdout,
+          ].join('\\n'),
+        }),
+      )
+      .catch((error) => panel.update({ markdown: 'exec failed: ' + error.message }));
+  });
+};
+`;
+
+type Started = { session: E2eSession; overlay: CdpPage };
+let started: Promise<Started> | undefined;
+
+function useSession(): Promise<Started> {
+  started ??= (async () => {
+    const session = await startE2eSession({
+      config: { plugins: { 'e2e-probe': { enabled: true, settings: { greeting: 'hello' } } } },
+      files: {
+        'plugins/e2e-probe/plugin.json': JSON.stringify({ name: 'E2E Probe', main: 'main.cjs' }),
+        'plugins/e2e-probe/main.cjs': PROBE_PLUGIN,
+      },
+    });
+    try {
+      return { session, overlay: await session.openPage(OVERLAY_PAGE) };
+    } catch (error) {
+      await session.dispose();
+      throw error;
+    }
+  })();
+  return started;
+}
+
+after(async () => {
+  const running = await started?.catch(() => undefined);
+  await running?.session.dispose();
+});
+
+type PanelState = {
+  title: string;
+  heading: string;
+  current: string;
+  items: string[];
+};
+
+test(
+  'a plugin action opens the plugin panel with subtitle context and child process output',
+  { timeout: TEST_TIMEOUT_MS },
+  async () => {
+    const { session, overlay } = await useSession();
+    const [previous, cue, next] = FIXTURE_CUES;
+    await session.mpv.command('seek', cue.at, 'absolute+exact');
+    await overlay.waitFor(
+      `document.getElementById('subtitleRoot').textContent === ${JSON.stringify(cue.text)}`,
+      { description: `overlay to show "${cue.text}"` },
+    );
+
+    await session.runAppCommand(
+      '--session-action',
+      JSON.stringify({
+        actionId: 'runPluginAction',
+        payload: { pluginActionId: 'e2e-probe/show' },
+      }),
+    );
+
+    const modal = await session.openPage('renderer/index.html?layer=modal');
+    const panel = await modal
+      .waitFor<PanelState | false>(
+        `(() => {
+        const root = document.getElementById('pluginPanelModal');
+        const body = document.getElementById('pluginPanelBody');
+        if (root.classList.contains('hidden') || !/exec[: ]/.test(body.textContent)) return false;
+        return {
+          title: document.getElementById('pluginPanelTitle').textContent,
+          heading: body.querySelector('h4')?.textContent ?? '',
+          current: body.querySelector('strong')?.textContent ?? '',
+          items: Array.from(body.querySelectorAll('li'), (item) => item.textContent),
+        };
+      })()`,
+        { description: 'plugin panel to show the finished probe result' },
+      )
+      .catch(async (error: unknown) => {
+        const body = await modal.evaluate<string>(
+          `document.getElementById('pluginPanelBody').textContent`,
+        );
+        throw new Error(`Plugin panel body: ${JSON.stringify(body)}`, { cause: error });
+      });
+
+    assert.deepEqual(panel, {
+      title: 'Probe',
+      heading: 'hello',
+      current: cue.text,
+      items: [
+        `before: ${previous.text}`,
+        `current: ${cue.text}`,
+        `after: ${next.text}`,
+        'exec: child-ok',
+      ],
+    });
+
+    // Closing hides the modal window (Linux destroys it), often before Chromium
+    // acknowledges the key event, so the send is not awaited. The plugin's log line
+    // proves the close reached main and aborted the handle.
+    void modal
+      .send('Input.dispatchKeyEvent', {
+        type: 'rawKeyDown',
+        key: 'Escape',
+        code: 'Escape',
+        windowsVirtualKeyCode: 27,
+      })
+      .catch(() => {});
+    await waitUntil(
+      () =>
+        fs.readFileSync(path.join(session.root, 'app.log'), 'utf8').includes('probe panel aborted'),
+      { description: 'closing the panel to abort the plugin handle in main' },
+    );
+  },
+);