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/mixin_base.py

@@ -0,0 +1,54 @@
+"""Shared base for the ``IrisAdapter`` mixin classes.
+
+``adapter.IrisAdapter`` is assembled from several small mixin classes
+(``inbound``, ``tool_frames``, ``push_frames``, ...) plus the core state and
+lifecycle in ``adapter`` itself. Each mixin references instance attributes and
+helper methods that are defined in the core class or in a *sibling* mixin, so a
+type checker analysing one mixin in isolation cannot see them.
+
+This base declares those shared names (as ``Any``) so static analysis resolves
+``self.<name>`` inside every mixin. The annotations carry no runtime effect;
+the real values are set in ``IrisAdapter.__init__`` and the real methods live
+in the core class / sibling mixins.
+"""
+
+from typing import Any
+
+
+class IrisAdapterBase:
+    """Declaration-only base for the ``IrisAdapter`` mixins (see module doc)."""
+
+    # -- shared state (set in ``IrisAdapter.__init__``) -------------------
+    _channels: Any
+    _devices: Any
+    _http_server: Any
+    _media: Any
+    _outbox: Any
+    _push: Any
+    _active_lane: Any
+    _last_message_id: Any
+    _last_push_at: Any
+    _pending_push: Any
+    _pending_pickers: Any
+    _prune_notified_at: Any
+    _typing_turns: Any
+    home_channel: Any
+    home_channel_name: Any
+
+    # -- shared helpers (core class or sibling mixins) --------------------
+    _broadcast_both: Any
+    _broadcast_or_log: Any
+    _channel_name: Any
+    _maybe_push: Any
+    _offer_media: Any
+    _parse_tool_line_or_block: Any
+    _push_summary: Any
+    _reply: Any
+    _schedule_thread_title_upgrade: Any
+
+    # -- provided by ``BasePlatformAdapter`` / core -----------------------
+    build_source: Any
+    handle_message: Any
+    send: Any
+    send_image: Any
+    send_slash_confirm: Any