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/search.py

@@ -116,7 +116,7 @@ def _row_to_hit(row: sqlite3.Row) -> dict[str, Any]:
     }
 
 
-def _fts_query(
+def _fts_query(  # noqa: PLR0913
     conn: sqlite3.Connection,
     query: str,
     scope: str,
@@ -153,7 +153,7 @@ def _fts_query(
     return [_row_to_hit(r) for r in rows]
 
 
-def _like_query(
+def _like_query(  # noqa: PLR0913
     conn: sqlite3.Connection,
     query: str,
     scope: str,
@@ -197,7 +197,7 @@ def _like_query(
     return [_row_to_hit(r) for r in rows]
 
 
-def search(
+def search(  # noqa: PLR0913
     db_path: Path,
     query: str,
     scope: str = "all",