feat: implement Helix-style modal buffer under DO-178C DAL-C

The repository was an unmodified ratatui component template: no editor
code, JSON5 config, and placeholder widgets. This establishes the first
working baseline — `moji <file>` opens a file into a ropey rope and edits
it with Helix selection-first semantics.

Requirements, implementation and tests land together because they must:
the traceability check rejects requirements with no implementation and
tests naming requirements that do not exist, so neither half is a valid
commit on its own.

Package renamed to mojibake-editor (mojibake was taken on crates.io);
binary is moji, library target stays mojibake.

Class:        New behaviour
Requirements: MJB-HLR-001..019, MJB-LLR-001..205
Derived:      MJB-DR-001..007 (DR-001 resolved, six open for review)
Verified:     cargo build; clippy --all-targets -D warnings clean;
              cargo test 294 passing; ./scripts/check-trace.sh 98/98/98;
              cargo package clean

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

1 files changed, 125 insertions, 0 deletions

diff --git a/src/components.rs b/src/components.rs
new file mode 100644
index 0000000..1771a17
--- /dev/null
+++ b/src/components.rs
@@ -0,0 +1,125 @@
+use crossterm::event::{KeyEvent, MouseEvent};
+use ratatui::{
+    Frame,
+    layout::{Rect, Size},
+};
+use tokio::sync::mpsc::UnboundedSender;
+
+use crate::{action::Action, config::Config, tui::Event};
+
+// MJB-LLR-204, MJB-HLR-019: the buffer is the only widget. The template's
+// `fps` and `home` modules were deleted, not merely unregistered.
+pub mod buffer;
+
+/// `Component` is a trait that represents a visual and interactive element of the user interface.
+///
+/// Implementors of this trait can be registered with the main application loop and will be able to
+/// receive events, update state, and be rendered on the screen.
+pub trait Component {
+    /// Register an action handler that can send actions for processing if necessary.
+    ///
+    /// # Arguments
+    ///
+    /// * `tx` - An unbounded sender that can send actions.
+    ///
+    /// # Returns
+    ///
+    /// * [`color_eyre::Result<()>`] - An Ok result or an error.
+    fn register_action_handler(&mut self, tx: UnboundedSender<Action>) -> color_eyre::Result<()> {
+        let _ = tx; // to appease clippy
+        Ok(())
+    }
+    /// Register a configuration handler that provides configuration settings if necessary.
+    ///
+    /// # Arguments
+    ///
+    /// * `config` - Configuration settings.
+    ///
+    /// # Returns
+    ///
+    /// * [`color_eyre::Result<()>`] - An Ok result or an error.
+    fn register_config_handler(&mut self, config: Config) -> color_eyre::Result<()> {
+        let _ = config; // to appease clippy
+        Ok(())
+    }
+    /// Initialize the component with a specified area if necessary.
+    ///
+    /// # Arguments
+    ///
+    /// * `area` - Rectangular area to initialize the component within.
+    ///
+    /// # Returns
+    ///
+    /// * [`color_eyre::Result<()>`] - An Ok result or an error.
+    fn init(&mut self, area: Size) -> color_eyre::Result<()> {
+        let _ = area; // to appease clippy
+        Ok(())
+    }
+    /// Handle incoming events and produce actions if necessary.
+    ///
+    /// # Arguments
+    ///
+    /// * `event` - An optional event to be processed.
+    ///
+    /// # Returns
+    ///
+    /// * [`color_eyre::Result<Option<Action>>`] - An action to be processed or none.
+    fn handle_events(&mut self, event: Option<Event>) -> color_eyre::Result<Option<Action>> {
+        let action = match event {
+            Some(Event::Key(key_event)) => self.handle_key_event(key_event)?,
+            Some(Event::Mouse(mouse_event)) => self.handle_mouse_event(mouse_event)?,
+            _ => None,
+        };
+        Ok(action)
+    }
+    /// Handle key events and produce actions if necessary.
+    ///
+    /// # Arguments
+    ///
+    /// * `key` - A key event to be processed.
+    ///
+    /// # Returns
+    ///
+    /// * [`color_eyre::Result<Option<Action>>`] - An action to be processed or none.
+    fn handle_key_event(&mut self, key: KeyEvent) -> color_eyre::Result<Option<Action>> {
+        let _ = key; // to appease clippy
+        Ok(None)
+    }
+    /// Handle mouse events and produce actions if necessary.
+    ///
+    /// # Arguments
+    ///
+    /// * `mouse` - A mouse event to be processed.
+    ///
+    /// # Returns
+    ///
+    /// * [`color_eyre::Result<Option<Action>>`] - An action to be processed or none.
+    fn handle_mouse_event(&mut self, mouse: MouseEvent) -> color_eyre::Result<Option<Action>> {
+        let _ = mouse; // to appease clippy
+        Ok(None)
+    }
+    /// Update the state of the component based on a received action. (REQUIRED)
+    ///
+    /// # Arguments
+    ///
+    /// * `action` - An action that may modify the state of the component.
+    ///
+    /// # Returns
+    ///
+    /// * [`color_eyre::Result<Option<Action>>`] - An action to be processed or none.
+    fn update(&mut self, action: Action) -> color_eyre::Result<Option<Action>> {
+        let _ = action; // to appease clippy
+        Ok(None)
+    }
+    /// Render the component on the screen. (REQUIRED)
+    ///
+    /// # Arguments
+    ///
+    /// * `f` - A frame used for rendering.
+    /// * `area` - The area in which the component should be drawn.
+    ///
+    /// # Returns
+    ///
+    /// * [`color_eyre::Result<()>`] - An Ok result or an error.
+    fn draw(&mut self, frame: &mut Frame, area: Rect) -> color_eyre::Result<()>;
+}