feat(text-injection): speed up ydotool injection with explicit --key-delay (#488)

Pass --key-delay 8 to ydotool type instead of relying on the default
(12-20ms). This makes text injection ~3x faster while keeping key-delay
above zero to avoid the Shift-leak bug where capital letters corrupt
following characters.

The delay is configurable via VOCALINUX_YDOTOOL_KEY_DELAY env var.

Note: --key-hold flag from the original issue does not exist in standard
ydotool (verified against official man page and v1.0.4 source).

Fixes #482

tests/test_text_injector.py

@@ -205,9 +205,9 @@ class TestTextInjector(unittest.TestCase):
             # Inject text
             injector.inject_text("Hello world")
 
-            # Verify ydotool was called correctly
+            # Verify ydotool was called correctly with timing parameters
             self.mock_subprocess.assert_any_call(
-                ["ydotool", "type", "Hello world"],
+                ["ydotool", "type", "--key-delay", "8", "Hello world"],
                 check=True,
                 stderr=subprocess.PIPE,
                 text=True,