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, 147 insertions, 0 deletions

diff --git a/src/buffer/command.rs b/src/buffer/command.rs
new file mode 100644
index 0000000..c5a2aef
--- /dev/null
+++ b/src/buffer/command.rs
@@ -0,0 +1,147 @@
+//! Editor commands — the values key bindings map to.
+//!
+//! Kept distinct from [`crate::action::Action`], which is application-level
+//! (tick, render, resize). A binding names a `Command`; the buffer executes it.
+//! `Quit` and `Suspend` appear here because the `Global` keymap is expressed in
+//! the same table and must be able to name them.
+
+use serde::{Deserialize, Serialize};
+use strum::Display;
+
+/// MJB-LLR-157: one unit variant per bound editor command, deserialized from
+/// the variant name so a TOML value like `"MoveCharLeft"` resolves directly.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Display, Serialize, Deserialize)]
+pub enum Command {
+    // --- Application (reachable from the `global` keymap) ---
+    Quit,
+    Suspend,
+
+    // --- Mode switching ---
+    NormalMode,
+    InsertMode,
+    SelectMode,
+    CommandMode,
+
+    // --- Character and line motion (MJB-HLR-006) ---
+    MoveCharLeft,
+    MoveCharRight,
+    MoveLineUp,
+    MoveLineDown,
+
+    // --- Word motion; these produce selections (MJB-HLR-007) ---
+    MoveNextWordStart,
+    MovePrevWordStart,
+    MoveNextWordEnd,
+    MoveNextLongWordStart,
+    MovePrevLongWordStart,
+    MoveNextLongWordEnd,
+
+    // --- Extending variants, used by select mode ---
+    ExtendCharLeft,
+    ExtendCharRight,
+    ExtendLineUp,
+    ExtendLineDown,
+    ExtendNextWordStart,
+    ExtendPrevWordStart,
+    ExtendNextWordEnd,
+
+    // --- Goto (MJB-HLR-008) ---
+    GotoFileStart,
+    GotoLastLine,
+    GotoLineStart,
+    GotoLineEnd,
+    GotoFirstNonWhitespace,
+
+    // --- Selection manipulation ---
+    ExtendLineBelow,
+    CollapseSelection,
+    FlipSelections,
+    SelectAll,
+
+    // --- Entering insert mode (MJB-HLR-009) ---
+    AppendMode,
+    InsertAtLineStart,
+    InsertAtLineEnd,
+    OpenBelow,
+    OpenAbove,
+
+    // --- Modification (MJB-HLR-010) ---
+    DeleteSelection,
+    ChangeSelection,
+    InsertNewline,
+    InsertTab,
+    DeleteCharBackward,
+    DeleteCharForward,
+    DeleteWordBackward,
+    KillToLineStart,
+
+    // --- Undo / redo (MJB-HLR-011) ---
+    Undo,
+    Redo,
+
+    // --- Scrolling and paging (MJB-HLR-013) ---
+    PageCursorHalfUp,
+    PageCursorHalfDown,
+    PageUp,
+    PageDown,
+
+    // --- Command line (MJB-HLR-016) ---
+    CommandSubmit,
+    CommandBackspace,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// MJB-LLR-157: a TOML value naming a variant deserializes to it, which is
+    /// what makes the keymap config-driven.
+    #[test]
+    fn mjb_llr_157_deserializes_from_the_variant_name() {
+        let cmd: Command = serde_json_free_parse("MoveCharLeft");
+        assert_eq!(cmd, Command::MoveCharLeft);
+        assert_eq!(serde_json_free_parse("Undo"), Command::Undo);
+        assert_eq!(
+            serde_json_free_parse("PageCursorHalfDown"),
+            Command::PageCursorHalfDown
+        );
+    }
+
+    #[test]
+    fn mjb_llr_157_unknown_command_name_is_an_error_not_a_panic() {
+        let err = toml::from_str::<Wrapper>("cmd = \"NoSuchCommand\"").unwrap_err();
+        assert!(
+            err.to_string().contains("NoSuchCommand"),
+            "error must name the offending value, got: {err}"
+        );
+    }
+
+    #[test]
+    fn mjb_llr_157_display_round_trips_through_deserialization() {
+        for cmd in [
+            Command::Quit,
+            Command::GotoFileStart,
+            Command::DeleteSelection,
+            Command::CommandSubmit,
+        ] {
+            assert_eq!(
+                serde_json_free_parse(&cmd.to_string()),
+                cmd,
+                "{cmd} must round-trip"
+            );
+        }
+    }
+
+    #[derive(Debug, serde::Deserialize)]
+    struct Wrapper {
+        cmd: Command,
+    }
+
+    /// Parse a bare command name the way the keymap table does.
+    fn serde_json_free_parse(name: &str) -> Command {
+        let doc = format!("cmd = \"{name}\"");
+        toml::from_str::<Wrapper>(&doc)
+            .unwrap_or_else(|e| panic!("{name} must parse: {e}"))
+            .cmd
+    }
+}