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/http_server.py

@@ -480,7 +480,7 @@ class HttpServer:
 
     # ── GET /v1/events (SSE) ──────────────────────────────────────────────
 
-    def _handle_sse(self, handler: BaseHTTPRequestHandler, device_id: str, parsed: Any) -> None:
+    def _handle_sse(self, handler: BaseHTTPRequestHandler, device_id: str, parsed: Any) -> None:  # noqa: PLR0912,PLR0915
         qs = parse_qs(parsed.query)
         cursor = _parse_cursor(qs.get("cursor", [None])[0], handler.headers.get("Last-Event-ID"))
         # Device registration (the HTTP equivalent of the WS hello upsert):
@@ -600,7 +600,7 @@ class HttpServer:
 
     # ── POST /v1/media (upload, docs/19 §19.15) ───────────────────────────
 
-    def _handle_media_upload(self, handler: BaseHTTPRequestHandler, device_id: str) -> None:
+    def _handle_media_upload(self, handler: BaseHTTPRequestHandler, device_id: str) -> None:  # noqa: PLR0911
         """Whole-file upload: metadata in headers, file bytes as the body.
 
         Mirrors the WS ``media.upload`` contract (docs/07 §7.2) in one
@@ -820,7 +820,7 @@ class _Handler(BaseHTTPRequestHandler):
             return
         _send_json(self, 404, {"error": "not found"})
 
-    def do_POST(self) -> None:  # noqa: N802
+    def do_POST(self) -> None:  # noqa: N802, PLR0911, PLR0912
         hs = self.server.http_server
         if not hs.enabled:
             _send_json(self, 503, {"error": "http leg disabled"})