Add REPL usage documentation and fix raw mode handling

- Fix raw mode enabling to not duplicate with UI border mode
- Add REPL_USAGE.md with comprehensive guide
- Add examples/repl_demo_terminal.py example script

REPL_USAGE.md

@@ -0,0 +1,116 @@
+# REPL Usage Guide
+
+The REPL (Read-Eval-Print Loop) effect provides an interactive command-line interface for controlling Mainline's pipeline in real-time.
+
+## How to Access the REPL
+
+### Method 1: Using CLI Arguments (Recommended)
+
+Run Mainline with the `repl` effect added to the effects list:
+
+```bash
+# With empty source (for testing)
+python mainline.py --pipeline-source empty --pipeline-effects repl
+
+# With headlines source (requires network)
+python mainline.py --pipeline-source headlines --pipeline-effects repl
+
+# With poetry source
+python mainline.py --pipeline-source poetry --pipeline-effects repl
+```
+
+### Method 2: Using a Preset
+
+Add a preset to your `~/.config/mainline/presets.toml` or `./presets.toml`:
+
+```toml
+[presets.repl]
+description = "Interactive REPL control"
+source = "headlines"
+display = "terminal"
+effects = ["repl"]
+viewport_width = 80
+viewport_height = 24
+```
+
+Then run:
+```bash
+python mainline.py --preset repl
+```
+
+### Method 3: Using Graph Config
+
+Create a TOML file (e.g., `repl_config.toml`):
+
+```toml
+source = "empty"
+display = "terminal"
+effects = ["repl"]
+```
+
+Then run:
+```bash
+python mainline.py --graph-config repl_config.toml
+```
+
+## REPL Commands
+
+Once the REPL is active, you can type commands:
+
+- **help** - Show available commands
+- **status** - Show pipeline status and metrics
+- **effects** - List all effects in the pipeline
+- **effect \<name\> \<on|off\>** - Toggle an effect
+- **param \<effect\> \<param\> \<value\>** - Set effect parameter
+- **pipeline** - Show current pipeline order
+- **clear** - Clear output buffer
+- **quit/exit** - Show exit message (use Ctrl+C to actually exit)
+
+## Keyboard Controls
+
+- **Enter** - Execute command
+- **Up/Down arrows** - Navigate command history
+- **Backspace** - Delete last character
+- **Ctrl+C** - Exit Mainline
+
+## Visual Features
+
+The REPL displays:
+- **HUD header** (top 3 lines): Shows FPS, frame time, command count, and output buffer size
+- **Content area**: Main content from the data source
+- **Separator line**: Visual divider
+- **REPL area**: Output buffer and input prompt
+
+## Example Session
+
+```
+MAINLINE REPL | FPS: 60.0 | 12.5ms
+COMMANDS: 3 | [2/3]
+OUTPUT: 5 lines
+────────────────────────────────────────
+Content from source appears here...
+More content...
+────────────────────────────────────────
+> help
+Available commands:
+  help - Show this help
+  status - Show pipeline status
+  effects - List all effects
+  effect <name> <on|off> - Toggle effect
+  param <effect> <param> <value> - Set parameter
+  pipeline - Show current pipeline order
+  clear - Clear output buffer
+  quit - Show exit message
+> effects
+Pipeline effects:
+  1. repl
+> effect repl off
+Effect 'repl' set to off
+```
+
+## Notes
+
+- The REPL effect needs a content source to overlay on (e.g., headlines, poetry, empty)
+- The REPL uses terminal display with raw input mode
+- Command history is preserved across sessions (up to 50 commands)
+- Pipeline mutations (enabling/disabling effects) are handled automatically