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.

tests/test_config_flow_pairing.py

@@ -15,10 +15,9 @@ import pytest
 from conftest import _SHIELD_DIR, _load_module
 from nsh.const import (
     CONF_ADB_ENABLED,
-    CONF_ADB_HOST,
     CONF_ADB_PORT,
     CONF_ADB_STORE_APP_INFO,
-    CONF_ANDROIDTV_ENTITY,
+    CONF_HOST,
     CONF_JELLYFIN_ENTITY,
     CONF_POWER_ENTITY,
     CONF_RECEIVER_ENTITY,
@@ -164,9 +163,8 @@ def _flow(user_input_host: str, monkeypatch=None):
     flow._user_data = {
         "name": "Nvidia Shield",
         CONF_ADB_ENABLED: True,
-        CONF_ADB_HOST: user_input_host,
+        CONF_HOST: user_input_host,
         CONF_ADB_PORT: 5555,
-        CONF_ANDROIDTV_ENTITY: "media_player.shield",
         CONF_JELLYFIN_ENTITY: "",
         CONF_RECEIVER_ENTITY: "",
         CONF_RECEIVER_INPUT: "",