refactor: remove legacy demo code, integrate metrics via pipeline context

- Remove ~700 lines of legacy code from app.py (run_demo_mode, run_pipeline_demo,
  run_preset_mode, font picker, effects picker)
- HUD now reads metrics from pipeline context (first-class citizen) with fallback
  to global monitor for backwards compatibility
- Add validate_signal_flow() for PureData-style type validation in presets
- Update MicSensor documentation (self-contained, doesn't use MicMonitor)
- Delete test_app.py (was testing removed legacy code)
- Update AGENTS.md with pipeline architecture documentation

tests/test_app.py

@@ -1,55 +0,0 @@
-"""
-Tests for engine.app module.
-"""
-
-from engine.app import _normalize_preview_rows
-
-
-class TestNormalizePreviewRows:
-    """Tests for _normalize_preview_rows function."""
-
-    def test_empty_rows(self):
-        """Empty input returns empty list."""
-        result = _normalize_preview_rows([])
-        assert result == [""]
-
-    def test_strips_left_padding(self):
-        """Left padding is stripped."""
-        result = _normalize_preview_rows(["  content", "  more"])
-        assert all(not r.startswith(" ") for r in result)
-
-    def test_preserves_content(self):
-        """Content is preserved."""
-        result = _normalize_preview_rows(["  hello world  "])
-        assert "hello world" in result[0]
-
-    def test_handles_all_empty_rows(self):
-        """All empty rows returns single empty string."""
-        result = _normalize_preview_rows(["", "   ", ""])
-        assert result == [""]
-
-
-class TestAppConstants:
-    """Tests for app module constants."""
-
-    def test_title_defined(self):
-        """TITLE is defined."""
-        from engine.app import TITLE
-
-        assert len(TITLE) > 0
-
-    def test_title_lines_are_strings(self):
-        """TITLE contains string lines."""
-        from engine.app import TITLE
-
-        assert all(isinstance(line, str) for line in TITLE)
-
-
-class TestAppImports:
-    """Tests for app module imports."""
-
-    def test_app_imports_without_error(self):
-        """Module imports without error."""
-        from engine import app
-
-        assert app is not None