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/dispatch.py

@@ -24,7 +24,7 @@ INBOUND_BURST = 40
 MAX_DEVICE_ID_LEN = 128
 
 
-async def dispatch_frame(adapter: Any, frame: protocol.Frame, device_id: str) -> None:
+async def dispatch_frame(adapter: Any, frame: protocol.Frame, device_id: str) -> None:  # noqa: PLR0912
     """Shared inbound frame dispatch (docs/19 §19.4). Unknown types are
     ignored (forward-compat)."""
     if frame.type == protocol.TYPE_MESSAGE_SEND: