fix(connection): a label dump that returned nothing is a FAILED fetch (t57 UNIT 0)

t59 F1 (HIGH). `ShieldAdbPoller.shell` swallows its error and answers None
(adb_client.py:144-155), and `_fetch_app_label` read that as "the dump
ran and found no label" → `cache_app_label(pkg, None)` → `('', stamp,
resolved=True)`. That is a DEFINITIVE negative for APP_LABEL_TTL = 24 h,
so after ONE transient failure `app_label()` never re-enqueued, exactly
one `pm dump` was ever attempted, and a later good dump could not
recover the label — `sensor.native_value` and `media_player.app_name`
stayed on the package id for every later render.

The dump now carries an exit marker (`; echo __NSH_EXIT:$?`), so the
three outcomes are distinguishable:

* output with an `application-label` → cache the label;
* output that ran and carried none (the marker is still there) → cache
  the empty string, the definitive negative that keeps t24 F1's
  zero-round-trip rule for a genuinely label-less package;
* no output at all (`None`, or blank) → cache NOTHING and let the next
  render re-enqueue, logged at debug.

The `resolved` flag goes with it: `cache_app_label` was its only writer
and always passed True, so an entry in `_app_labels` is now simply "a
fetch RAN" and the dead third field (`(label, stamp, True)`) is gone.

Tests drive the FakePoller-returns-None AND returns-'' shapes through
the sensor and through `media_player.app_name`, plus the label-less
package that must still be negatively cached. The stub poller now models
both real outcomes of a dump (an answer, or a transport that delivered
nothing) instead of conflating them — which is why the old tests could
only pin the raising-shell branch that already worked. 4 of the new
tests fail on the pre-fix tree, showing `('', stamp, True)` stamped.

Also in this commit:
- t59 F3: running_apps.py set `app["state"] = "paused"` twice — one line.
- t59 F4: no captured fixture contains state=6, so the "the Shield's own
  sessions report 6 like stock Android" wording in README and the
  media_session/media_player docstrings is reworded as INFERRED from the
  `state=PlaybackState {state=N, …}` line shape. A `git notes` entry on
  07682ff records the same correction against the t55 commit body.

tests/test_connection.py

@@ -18,7 +18,7 @@ from nsh.connection import (
     ShieldConnectionManager,
     StateStore,
 )
-from nsh.const import ADB_POLL_INTERVAL
+from nsh.const import ADB_POLL_INTERVAL, APP_LABEL_EXIT_MARKER
 
 
 def test_connection_module_depends_on_no_homeassistant():
@@ -310,8 +310,11 @@ class TestAppLabelCache:
 
         await asyncio.sleep(0)  # drain the fetch task
 
+        # t57: the dump carries the exit marker too, so a blank answer can be
+        # told from a FAILED fetch; the package is still quoted
         assert manager._poller.shell_calls == [
-            f"pm dump {shlex.quote(wicked)} 2>/dev/null | head -300"
+            f"pm dump {shlex.quote(wicked)} 2>/dev/null | head -300; "
+            f"echo {APP_LABEL_EXIT_MARKER}$?"
         ]