fix: resolve terminal display wobble and effect dimension stability

- Fix TerminalDisplay: add screen clear each frame (cursor home + erase down)
- Fix CameraStage: use set_canvas_size instead of read-only viewport properties
- Fix Glitch effect: preserve visible line lengths, remove cursor positioning
- Fix Fade effect: return original line when fade=0 instead of empty string
- Fix Noise effect: use input line length instead of terminal_width
- Remove HUD effect from all presets (redundant with border FPS display)
- Add regression tests for effect dimension stability
- Add docs/ARCHITECTURE.md with Mermaid diagrams
- Add mise tasks: diagram-ascii, diagram-validate, diagram-check
- Move markdown docs to docs/ (ARCHITECTURE, Refactor, hardware specs)
- Remove redundant requirements files (use pyproject.toml)
- Add *.dot and *.png to .gitignore

Closes #25

presets.toml

@@ -8,6 +8,60 @@
 #   - ~/.config/mainline/presets.toml
 #   - ./presets.toml (local override)
 
+# ============================================
+# TEST PRESETS
+# ============================================
+
+[presets.test-single-item]
+description = "Test: Single item to isolate rendering stage issues"
+source = "empty"
+display = "terminal"
+camera = "feed"
+effects = []
+camera_speed = 0.1
+viewport_width = 80
+viewport_height = 24
+
+[presets.test-single-item-border]
+description = "Test: Single item with border effect only"
+source = "empty"
+display = "terminal"
+camera = "feed"
+effects = ["border"]
+camera_speed = 0.1
+viewport_width = 80
+viewport_height = 24
+
+[presets.test-headlines]
+description = "Test: Headlines from cache with border effect"
+source = "headlines"
+display = "terminal"
+camera = "feed"
+effects = ["border"]
+camera_speed = 0.1
+viewport_width = 80
+viewport_height = 24
+
+[presets.test-headlines-noise]
+description = "Test: Headlines from cache with noise effect"
+source = "headlines"
+display = "terminal"
+camera = "feed"
+effects = ["noise"]
+camera_speed = 0.1
+viewport_width = 80
+viewport_height = 24
+
+[presets.test-demo-effects]
+description = "Test: All demo effects with terminal display"
+source = "headlines"
+display = "terminal"
+camera = "feed"
+effects = ["noise", "fade", "firehose"]
+camera_speed = 0.3
+viewport_width = 80
+viewport_height = 24
+
 # ============================================
 # DATA SOURCE GALLERY
 # ============================================