feat(presets): add TOML preset loader with validation

- Convert presets from YAML to TOML format (no external dep)
- Add DEFAULT_PRESET fallback for graceful degradation
- Add validate_preset() for preset validation
- Add validate_signal_path() for circular dependency detection
- Add generate_preset_toml() for skeleton generation
- Use tomllib (Python 3.11+ stdlib)

presets.toml

@@ -0,0 +1,106 @@
+# Mainline Presets Configuration
+# Human- and machine-readable preset definitions
+#
+# Format: TOML
+# Usage: mainline --preset <name>
+#
+# Built-in presets can be overridden by user presets in:
+#   - ~/.config/mainline/presets.toml
+#   - ./presets.toml (local override)
+
+[presets.demo]
+description = "Demo mode with effect cycling and camera modes"
+source = "headlines"
+display = "pygame"
+camera = "vertical"
+effects = ["noise", "fade", "glitch", "firehose", "hud"]
+viewport_width = 80
+viewport_height = 24
+camera_speed = 1.0
+firehose_enabled = true
+
+[presets.poetry]
+description = "Poetry feed with subtle effects"
+source = "poetry"
+display = "pygame"
+camera = "vertical"
+effects = ["fade", "hud"]
+viewport_width = 80
+viewport_height = 24
+camera_speed = 0.5
+firehose_enabled = false
+
+[presets.pipeline]
+description = "Pipeline visualization mode"
+source = "pipeline"
+display = "terminal"
+camera = "trace"
+effects = ["hud"]
+viewport_width = 80
+viewport_height = 24
+camera_speed = 1.0
+firehose_enabled = false
+
+[presets.websocket]
+description = "WebSocket display mode"
+source = "headlines"
+display = "websocket"
+camera = "vertical"
+effects = ["noise", "fade", "glitch", "hud"]
+viewport_width = 80
+viewport_height = 24
+camera_speed = 1.0
+firehose_enabled = false
+
+[presets.sixel]
+description = "Sixel graphics display mode"
+source = "headlines"
+display = "sixel"
+camera = "vertical"
+effects = ["noise", "fade", "glitch", "hud"]
+viewport_width = 80
+viewport_height = 24
+camera_speed = 1.0
+firehose_enabled = false
+
+[presets.firehose]
+description = "High-speed firehose mode"
+source = "headlines"
+display = "pygame"
+camera = "vertical"
+effects = ["noise", "fade", "glitch", "firehose", "hud"]
+viewport_width = 80
+viewport_height = 24
+camera_speed = 2.0
+firehose_enabled = true
+
+# Sensor configuration (for future use with param bindings)
+[sensors.mic]
+enabled = false
+threshold_db = 50.0
+
+[sensors.oscillator]
+enabled = false
+waveform = "sine"
+frequency = 1.0
+
+# Effect configurations
+[effect_configs.noise]
+enabled = true
+intensity = 1.0
+
+[effect_configs.fade]
+enabled = true
+intensity = 1.0
+
+[effect_configs.glitch]
+enabled = true
+intensity = 0.5
+
+[effect_configs.firehose]
+enabled = true
+intensity = 1.0
+
+[effect_configs.hud]
+enabled = true
+intensity = 1.0