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, 228 insertions, 0 deletions

diff --git a/src/buffer/history.rs b/src/buffer/history.rs
new file mode 100644
index 0000000..e4668fc
--- /dev/null
+++ b/src/buffer/history.rs
@@ -0,0 +1,228 @@
+//! Undo/redo history (MJB-HLR-011).
+//!
+//! Each committed edit stores the pair (forward transaction, inverse
+//! transaction). `cursor` is the number of entries currently *applied*, so
+//! entries at or beyond it have been reverted and are available to redo.
+//!
+//! Undo at the start and redo at the end are no-ops, not errors — reaching
+//! either end is ordinary use, not a fault (MJB-LLR-052, MJB-LLR-053).
+
+use super::transaction::Transaction;
+
+#[derive(Debug, Clone)]
+struct Entry {
+    forward: Transaction,
+    inverse: Transaction,
+    /// Identifies the buffer state this entry produces. Never reused.
+    id: usize,
+}
+
+#[derive(Debug, Clone, Default)]
+pub struct History {
+    entries: Vec<Entry>,
+    /// Number of entries applied; also the index of the next redo.
+    cursor: usize,
+    /// Monotonic source of entry ids.
+    ///
+    /// Deliberately *not* the same thing as `cursor`. Using stack depth to
+    /// identify a state is wrong: saving at depth 3, undoing, then making a
+    /// different edit returns to depth 3 while the content differs, so a
+    /// depth comparison would report the buffer clean when it is not.
+    next_id: usize,
+}
+
+impl History {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// MJB-LLR-051: record an applied edit.
+    ///
+    /// Anything previously undone is discarded — committing a new edit after an
+    /// undo abandons the branch that was reverted.
+    pub fn commit(&mut self, forward: Transaction, inverse: Transaction) {
+        self.entries.truncate(self.cursor);
+        self.next_id += 1;
+        self.entries.push(Entry {
+            forward,
+            inverse,
+            id: self.next_id,
+        });
+        self.cursor = self.entries.len();
+    }
+
+    /// MJB-LLR-052: the transaction that reverts the most recent edit, or
+    /// `None` when nothing remains to undo.
+    pub fn undo(&mut self) -> Option<&Transaction> {
+        if self.cursor == 0 {
+            return None;
+        }
+        self.cursor -= 1;
+        Some(&self.entries[self.cursor].inverse)
+    }
+
+    /// MJB-LLR-053: the transaction that reapplies the most recently undone
+    /// edit, or `None` when nothing remains to redo.
+    pub fn redo(&mut self) -> Option<&Transaction> {
+        if self.cursor >= self.entries.len() {
+            return None;
+        }
+        let t = &self.entries[self.cursor].forward;
+        self.cursor += 1;
+        Some(t)
+    }
+
+    /// Identifies the current buffer state.
+    ///
+    /// Zero means pristine — no edit applied. Otherwise it is the id of the
+    /// most recently applied entry. Two calls return the same value exactly
+    /// when the buffer content is the same, which is what lets "modified" be
+    /// *derived* rather than latched; see
+    /// [`super::document::Document::is_modified`].
+    pub fn revision(&self) -> usize {
+        match self.cursor {
+            0 => 0,
+            n => self.entries[n - 1].id,
+        }
+    }
+
+    pub fn can_undo(&self) -> bool {
+        self.cursor > 0
+    }
+
+    pub fn can_redo(&self) -> bool {
+        self.cursor < self.entries.len()
+    }
+
+    pub fn len(&self) -> usize {
+        self.entries.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.entries.is_empty()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use ropey::Rope;
+
+    use super::*;
+    use crate::buffer::transaction::Transaction;
+
+    fn edit(text: &str, from: usize, to: usize, ins: Option<&str>) -> (Transaction, Transaction) {
+        let r = Rope::from_str(text);
+        let t = Transaction::change(&r, [(from, to, ins.map(str::to_owned))]);
+        let inv = Transaction::new(t.changes.invert(&r));
+        (t, inv)
+    }
+
+    #[test]
+    fn mjb_llr_052_undo_past_start_is_a_noop() {
+        let mut h = History::new();
+        assert!(h.undo().is_none());
+        assert!(!h.can_undo());
+        // Repeated attempts must stay safe, not underflow the cursor.
+        assert!(h.undo().is_none());
+        assert!(h.undo().is_none());
+    }
+
+    #[test]
+    fn mjb_llr_053_redo_past_end_is_a_noop() {
+        let mut h = History::new();
+        let (f, i) = edit("abc", 0, 1, None);
+        h.commit(f, i);
+        assert!(h.redo().is_none(), "nothing has been undone yet");
+        assert!(!h.can_redo());
+    }
+
+    #[test]
+    fn mjb_llr_051_commit_then_undo_then_redo() {
+        let mut h = History::new();
+        let (f, i) = edit("abc", 0, 1, None);
+        h.commit(f, i);
+
+        assert!(h.can_undo());
+        assert!(h.undo().is_some());
+        assert!(!h.can_undo());
+        assert!(h.can_redo());
+        assert!(h.redo().is_some());
+        assert!(!h.can_redo());
+    }
+
+    #[test]
+    fn mjb_llr_051_commit_after_undo_discards_the_redo_branch() {
+        let mut h = History::new();
+        let (f1, i1) = edit("abc", 0, 1, None);
+        let (f2, i2) = edit("bc", 0, 1, None);
+        h.commit(f1, i1);
+        h.commit(f2, i2);
+
+        h.undo();
+        assert!(h.can_redo());
+
+        let (f3, i3) = edit("bc", 1, 2, None);
+        h.commit(f3, i3);
+        assert!(!h.can_redo(), "the undone branch must be discarded");
+        assert_eq!(h.len(), 2);
+    }
+
+    /// MJB-LLR-116: a revision identifies *content*, not stack depth.
+    ///
+    /// Regression guard for the trap this replaced: save at depth 2, undo, then
+    /// make a different edit. The cursor returns to 2, but the buffer no longer
+    /// matches what was saved, so the revision must differ.
+    #[test]
+    fn mjb_llr_116_revision_is_not_stack_depth() {
+        let mut h = History::new();
+        let (f1, i1) = edit("abcdef", 0, 1, None);
+        let (f2, i2) = edit("bcdef", 0, 1, None);
+        h.commit(f1, i1);
+        h.commit(f2, i2);
+
+        let saved = h.revision();
+
+        h.undo();
+        assert_ne!(h.revision(), saved, "undo leaves a different state");
+
+        // A different second edit, landing at the same stack depth.
+        let (f3, i3) = edit("bcdef", 1, 2, None);
+        h.commit(f3, i3);
+        assert_eq!(h.len(), 2, "same depth as when we saved");
+        assert_ne!(
+            h.revision(),
+            saved,
+            "same depth, different content: must not look saved"
+        );
+    }
+
+    #[test]
+    fn mjb_llr_116_revision_is_zero_when_pristine_and_returns_on_undo() {
+        let mut h = History::new();
+        assert_eq!(h.revision(), 0);
+
+        let (f, i) = edit("abc", 0, 1, None);
+        h.commit(f, i);
+        let after = h.revision();
+        assert_ne!(after, 0);
+
+        h.undo();
+        assert_eq!(h.revision(), 0, "undoing to pristine returns to revision 0");
+        h.redo();
+        assert_eq!(h.revision(), after, "redo restores the same revision");
+    }
+
+    #[test]
+    fn mjb_llr_052_undo_walks_back_in_order() {
+        let mut h = History::new();
+        for n in 0..3 {
+            let (f, i) = edit("abcdef", n, n + 1, None);
+            h.commit(f, i);
+        }
+        assert_eq!(h.len(), 3);
+        for _ in 0..3 {
+            assert!(h.undo().is_some());
+        }
+        assert!(h.undo().is_none(), "exhausted");
+    }
+}