refactor(config): one Shield IP field for both transports (t41)

The two address fields were the same Shield spelled twice, and every bug in the
zeroconf/IP-change path came from keeping them in sync. One field now:

* const.CONF_HOST replaces CONF_ADB_HOST / CONF_PROTOCOL_HOST in the setup form,
  the options page, the pairing forms, the reconfigure/reauth seeding and the
  description copy. ADB port 5555 stays the only port field;
* helpers.entry_host(entry) is the single resolver (options first, then data);
  resolve_adb_config and the protocol settings both go through it, so both
  transports read the same value at call time (the t32 F1 factory re-read keeps
  working: the NEXT built poller sees the new address);
* helpers.seed_host / seed_adb_enabled own the YAML seed's shape rule. A block
  may use `host` or the legacy protocol_host/adb_host keys — t41 keeps the old
  spelling working — and t32 F4's outcome is unchanged: a protocol-only block
  leaves ADB off, an address (or an explicit adb_enabled) turns it on;
* the zeroconf IP-change path collapses the t21-F2 per-host dance into one rule:
  the discovery address IS the entry's address, so the entry is updated when it
  differs and left alone when it matches. The blank-self-heal and
  deliberately-different-ADB-host cases are gone with the duplication — a
  second address field cannot express them anymore, and the tests now pin that
  (previously "an explicit override is never clobbered");
* the options page exposes the protocol toggle it never had (the page could turn
  ADB off but not the protocol);
* button.py (granted to this task) reads the entry's own address instead of
  another integration's entity.

The v3 entry migration that maps existing adb_host/protocol_host onto CONF_HOST
and drops the androidtv key lands in the next commit; HA runs migrations before
setup, so an existing entry is never read with the old shape.

Verified: 311 passed / 2 skipped, 312 / 1 with androidtvremote2, ruff clean,
drift engine exit 0 on HEAD. The UI-copy contract tests (every rendered field
has a label in strings.json AND translations/en.json, key trees identical, host
errors name the field) drove the string edits.

custom_components/nvidia_shield/button.py

@@ -11,12 +11,11 @@ from homeassistant.helpers.entity_platform import AddEntitiesCallback
 from .adb_session import AdbMediaSessionPoller, ensure_adb_key
 from .const import (
     CONF_ADB_ENABLED,
-    CONF_ADB_HOST,
     CONF_ADB_PORT,
-    CONF_PROTOCOL_HOST,
     DEFAULT_ADB_PORT,
     DOMAIN,
 )
+from .helpers import entry_host
 
 _LOGGER = logging.getLogger(__name__)
 
@@ -54,11 +53,9 @@ class NvidiaShieldPairAdbButton(ButtonEntity):
 
     async def async_press(self) -> None:
         """Attempt ADB connection. The Shield will show a pairing prompt."""
-        # t41: the address is ours — never derived from another integration's
-        # entity. The ADB field leads, then the protocol field (same Shield).
-        host = self._entry.options.get(CONF_ADB_HOST) or self._entry.options.get(
-            CONF_PROTOCOL_HOST
-        )
+        # t41: the entry's own single address — never derived from another
+        # integration's entity.
+        host = entry_host(self._entry)
         port = self._entry.options.get(CONF_ADB_PORT, DEFAULT_ADB_PORT)
 
         if not host: