test(files): cover text-file edit round trip

Opens server.cfg through the file-row name button, drives the CM6
controller via window.__filesEditor.setContent, clicks save, and
asserts both that the routed modal closes and that the new bytes
landed on disk under overlay_root. Guards against four classes of
regression at once: the /files/edit fragment delivering the wrong
data-rel-path, editor.js's htmx:afterSwap re-init failing to wire
__filesEditor, routedSaveClicked stopping short of closeRouted(),
and the /files/save endpoint failing to persist.

Adds a `_wait_for_routed_editor` helper centered on `.cm-content`
inside `#files-editor-fragment` — the textarea itself is
display:none after CM6 mounts, so to_be_visible on the textarea
would always fail; the cm-content surface is the real "editor is
ready" signal.

Per docs/superpowers/plans/2026-05-17-files-overlay-e2e-handoff.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

l4d2web/tests/e2e/test_files_overlay.py

@@ -0,0 +1,75 @@
+"""End-to-end Playwright tests for the files-overlay file manager.
+
+The files_overlay_server fixture (conftest.py) seeds user "alice"/"secret",
+a files-type Overlay owned by alice, and an overlay root pre-populated
+with one editable text file, one binary file, and one nested folder.
+Each test logs in, opens /overlays/<id>, drives the UI through real
+clicks + HTMX swaps, and asserts on DOM + filesystem state.
+
+Running locally: `uv run pytest -m e2e l4d2web/tests/e2e/test_files_overlay.py`.
+Requires `uv run playwright install chromium` once.
+
+Pattern model: test_editor.py (CM6 controller flow) + the handoff doc at
+docs/superpowers/plans/2026-05-17-files-overlay-e2e-handoff.md.
+"""
+from __future__ import annotations
+
+import pytest
+from playwright.sync_api import Page, expect
+
+from .conftest import login
+
+pytestmark = pytest.mark.e2e
+
+
+def _open_overlay(page: Page, base_url: str, overlay_id: int) -> None:
+    login(page, base_url)
+    page.goto(f"{base_url}/overlays/{overlay_id}")
+
+
+def _wait_for_routed_editor(page: Page) -> None:
+    """Wait until the routed modal's CM6 surface is mounted.
+
+    The textarea itself is `display:none` (editor.css pre-hides + CM6's
+    mount sets `style.display = "none"`), so we cannot assert visibility
+    on it directly. The `.cm-content` element appearing under
+    `#files-editor-fragment` is the canonical "editor is ready, the
+    `window.__filesEditor` controller is wired" signal.
+    """
+    expect(page.locator("#files-editor-fragment")).to_be_visible(timeout=5000)
+    expect(page.locator("#files-editor-fragment .cm-content")).to_be_visible(timeout=5000)
+
+
+def test_edit_text_file_save_round_trip(page: Page, files_overlay_server) -> None:
+    """Open server.cfg via the file-row name button, drive the CM6
+    controller to a new doc, save, and assert the disk reflects it.
+    Exercises the full edit path: row click → openRouted → htmx swap →
+    CM6 mount → save POST → modal close. Failure modes guarded against:
+      * /overlays/<id>/files/edit returns the wrong fragment (textarea
+        gets the wrong data-rel-path, or none).
+      * editor.js's htmx:afterSwap re-init doesn't fire / fails — then
+        window.__filesEditor stays unset and the save reads the stale
+        textarea.value instead of the user's edits.
+      * routedSaveClicked stops short of closeRouted() — modal stays
+        open and the test's hidden-assertion catches it.
+      * The POST /files/save endpoint doesn't actually write — covered
+        by reading the file back from disk.
+    """
+    base = files_overlay_server["base_url"]
+    overlay_id = files_overlay_server["overlay_id"]
+    overlay_root = files_overlay_server["overlay_root"]
+    _open_overlay(page, base, overlay_id)
+
+    page.click('button.file-tree-name-button[data-target-path="server.cfg"]')
+    _wait_for_routed_editor(page)
+    # Sanity: the modal opened for the file we asked for, not some other.
+    assert page.locator('textarea[data-rel-path="server.cfg"]').count() == 1
+
+    new_content = 'hostname "edited by e2e"\nmp_gamemode coop\n'
+    page.evaluate("(text) => window.__filesEditor.setContent(text)", new_content)
+
+    page.click(".files-editor-save")
+
+    # Modal close is async (await postJson then closeRouted()).
+    expect(page.locator("#modal-container")).to_be_hidden(timeout=5000)
+    assert (overlay_root / "server.cfg").read_text() == new_content