Split adapter.py monolith into focused modules; restore Ruff complexity defaults (issue #12)

adapter.py was a 3,493-line monolith. Split it into focused modules with
clear separation of responsibilities, bringing it down to ~857 lines:

- Module-level helpers: hooks, classify, pickers, commands, setup,
  defaults, secrets
- Frame-handler mixins: inbound, tool_frames, push_frames, media_frames,
  picker_frames, channel_frames, query_frames
- mixin_base: IrisAdapterBase (declaration-only base for shared attrs)
- adapter.py now holds only IrisAdapter (the composition of the 7 mixins
  + BasePlatformAdapter), register(), and test-facing re-exports

The mixins come before BasePlatformAdapter in the MRO so their methods
override the base; super() calls (e.g. send_image) still resolve to
BasePlatformAdapter. No circular imports; dispatch.py and http_server.py
(instance-method callers) are unaffected.

Ruff complexity ceilings (PLR0911/0912/0913/0915) restored to Ruff's
built-in defaults (12/50/6/5) instead of "just above the current maxima",
which ratchets the bar down as code grows. The existing genuinely-complex
functions (frame builders mirroring the wire schema, the QR matrix builder,
the dispatch table) carry an explicit `# noqa: PLR09xx` marking them as
reviewed, frozen exceptions; new code is held to the default ceilings.

All 125 tests green (94 test_android + 31 test_android_http); no new ruff
errors introduced.

gateway-plugin/push.py

@@ -63,7 +63,7 @@ class PushBackend:
         """True when the backend has credentials to send with."""
         raise NotImplementedError
 
-    async def send(
+    async def send(  # noqa: PLR0913
         self,
         *,
         device_id: str,
@@ -126,7 +126,7 @@ class FcmBackend(PushBackend):
         self._sa_failed = True
         return None
 
-    async def _authorization(self, client: httpx.AsyncClient) -> str | None:
+    async def _authorization(self, client: httpx.AsyncClient) -> str | None:  # noqa: PLR0911
         """Bearer token: the legacy server key, or a cached service-account
         OAuth2 access token (JWT-bearer grant, minted with PyJWT)."""
         if self._server_key:
@@ -187,7 +187,7 @@ class FcmBackend(PushBackend):
                 self._token_expiry = now + 3600.0
         return token
 
-    async def send(
+    async def send(  # noqa: PLR0913
         self,
         *,
         device_id: str,
@@ -277,7 +277,7 @@ class NtfyBackend(PushBackend):
     def configured(self) -> bool:
         return bool(self._topic)
 
-    async def send(
+    async def send(  # noqa: PLR0913
         self,
         *,
         device_id: str,
@@ -315,7 +315,7 @@ class NtfyBackend(PushBackend):
         return True
 
 
-def build_push_backend(
+def build_push_backend(  # noqa: PLR0913
     name: str | None,
     *,
     fcm_service_account: str | None = None,