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/defaults.py

@@ -0,0 +1,14 @@
+"""Platform defaults (config.yaml ``extra`` / env fallbacks)."""
+
+DEFAULT_HOST = "127.0.0.1"
+DEFAULT_PORT = 8790
+DEFAULT_HTTP_PORT = 8791  # docs/19: HTTP fallback leg
+DEFAULT_HOME_CHANNEL = "default"
+DEFAULT_HOME_CHANNEL_NAME = "Default"
+DEFAULT_PUSH_BACKEND = "ntfy"
+DEFAULT_OUTBOX_RETENTION_HOURS = 72
+DEFAULT_MAX_UPLOAD_BYTES = 100 * 1024 * 1024  # 100 MB
+
+
+def _truthy(value: str | None) -> bool:
+    return (value or "").strip().lower() in {"1", "true", "yes", "on"}