docs(testing[nav]): promote testing/ to first-class nav; drop empty libtmux.test page

why: api/testing/ was buried two levels deep (homepage → API Reference →
Testing card). The pytest plugin is a major feature that deserves direct
homepage visibility. Separately, libtmux.test.md generated an empty
page because test/__init__.py has only a docstring — all actual API is
already documented on the 5 submodule leaf pages.

what:
- Add Testing card to docs/index.md homepage grid (grid 1 1 2 2 → 1 1 3 3)
- Add api/testing/index to root toctree so Testing appears in sidebar
  alongside API Reference, Topics, Project
- Remove docs/api/testing/test-helpers/libtmux.test.md (empty automodule)
- Remove libtmux.test card and toctree entry from test-helpers/index.md
- Add redirect api/testing/test-helpers/libtmux.test → test-helpers/index

Author: tony

docs/api/testing/test-helpers/index.md

@@ -7,11 +7,6 @@ Utilities for writing reliable tests against libtmux and downstream code that us
 ::::{grid} 1 2 3 3
 :gutter: 2 2 3 3
 
-:::{grid-item-card} libtmux.test
-:link: libtmux.test
-:link-type: doc
-Base test module with common setup utilities.
-:::
 :::{grid-item-card} Constants
 :link: constants
 :link-type: doc
@@ -44,7 +39,6 @@ Context managers for ephemeral tmux resources.
 :hidden:
 :maxdepth: 1
 
-libtmux.test
 constants
 environment
 random

docs/api/testing/test-helpers/libtmux.test.md

@@ -5,7 +5,7 @@
 Typed Python API for [tmux](https://github.com/tmux/tmux). Control
 servers, sessions, windows, and panes as Python objects.
 
-::::{grid} 1 1 2 2
+::::{grid} 1 1 3 3
 :gutter: 2 2 3 3
 
 :::{grid-item-card} Quickstart
@@ -26,6 +26,12 @@ Architecture, traversal, filtering, and automation patterns.
 Every public class, function, and exception.
 :::
 
+:::{grid-item-card} Testing
+:link: api/testing/index
+:link-type: doc
+pytest plugin and test helpers for isolated tmux environments.
+:::
+
 :::{grid-item-card} Contributing
 :link: project/index
 :link-type: doc
@@ -93,6 +99,7 @@ def test_my_tool(session):
 quickstart
 topics/index
 api/index
+api/testing/index
 internals/index
 project/index
 history

docs/index.md

@@ -53,3 +53,5 @@
 "api/public-api.md" "project/public-api.md"
 "api/compatibility.md" "project/compatibility.md"
 "api/deprecations.md" "project/deprecations.md"
+# libtmux.test package page removed — empty __init__.py, no members to document
+"api/testing/test-helpers/libtmux.test.md" "api/testing/test-helpers/index.md"