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, 182 insertions, 8 deletions

diff --git a/README.md b/README.md
index cfd2ad9..fa71266 100644
--- a/README.md
+++ b/README.md
@@ -1,9 +1,183 @@
 # mojibake - 文字化け
----
-
-## TODO
-- [ ] setup ratatui component framework base
-- [ ] setup core buffer
-- [ ] setup feature type system
-- [ ] setup testing/documentation rules
-- [ ] setup license
+
+A terminal text editor with Helix-style modal editing.
+
+> [!WARNING]
+> **Early development — not ready for daily use.**
+>
+> There is no yank/paste, no search, and no syntax highlighting. Undo works one
+> keystroke at a time rather than per edit. Treat this as a working foundation,
+> not an editor you should trust with real files yet. See [Status](#status) for
+> exactly what does and does not work.
+
+```sh
+cargo install --path .
+moji <file>
+```
+
+The crate is `mojibake-editor` (the name `mojibake` was already taken on
+crates.io by an unrelated encoder); the command is `moji`.
+
+A path that does not exist is created on write. With no path, `moji` opens an
+empty scratch buffer.
+
+## Status
+
+### Works
+
+- Opens, edits and saves UTF-8 files; UTF-16 with a BOM round-trips
+- CRLF and CR line endings are detected and preserved
+- Helix selection-first editing: `w` selects a word, so `wd` deletes one
+- Motions, goto, insert-entry, delete/change, undo/redo, paging
+- Counts (`5l`, `2w`)
+- Config-driven modal keymap in TOML; chords resolve without a timeout
+- Writes through symlinks, preserves permissions, restores on failure
+- Rendering is O(viewport) — a 200k-line file scrolls without lag
+
+### Does not work yet
+
+- **No yank or paste.** `y`, `p`, `P` are unbound.
+- **No search.** `/`, `?`, `n`, `N` are unbound.
+- **No `f`/`t` motions.** They need pending-argument capture, which the keymap
+  cannot express yet.
+- **Undo is per keystroke.** Typing `abc` in insert mode costs three undos;
+  Helix commits one checkpoint when you leave insert mode.
+- **`:w <path>` ignores its argument** and writes to the original path.
+- **One file at a time.** No buffer list, no splits.
+- No syntax highlighting, no LSP, no multiple cursors, no soft wrap.
+
+## Keybindings
+
+Bindings follow the [Helix default keymap](https://docs.helix-editor.com/keymap.html)
+and live in `.config/config.toml`. Defaults ship compiled in; user settings
+override them **per individual binding**, so rebinding one key keeps every
+default you did not mention.
+
+Editing is **selection-first**, like Helix and unlike Vim: a motion leaves a
+selection and an operator acts on it. There is no operator-pending state.
+
+| Mode | Keys |
+|---|---|
+| Motion | `h` `j` `k` `l`, `w` `b` `e`, `W` `B` `E` |
+| Goto | `gg` `ge` `gh` `gl` `gs` |
+| Selection | `x` `;` `%` `v` `Alt-;` |
+| Insert | `i` `a` `I` `A` `o` `O` |
+| Change | `d` `c`, `u` undo, `U` redo |
+| Scroll | `Ctrl-u` `Ctrl-d` half page, `Ctrl-b` `Ctrl-f` full page |
+| Command | `:w` `:q` `:wq` `:x` `:q!` `:w!` |
+
+Counts work as a prefix: `5l`, `12j`.
+
+## Configuration
+
+TOML only, at `$MOJIBAKE_CONFIG/config.toml`. See `.config/config.toml` for the
+annotated defaults.
+
+```toml
+[editor]
+scrolloff = 5
+insert_final_newline = true
+
+[keybindings.normal]
+"<g><g>" = "GotoFileStart"
+"<ctrl-d>" = "PageCursorHalfDown"
+```
+
+## Roadmap
+
+Ordered by what most blocks real use.
+
+### 1 — Blocking daily use
+
+- [ ] Yank and paste: `y`, `p`, `P`, plus a register
+- [ ] Undo checkpoints — group an insert session into one step
+      (Helix commits on leaving insert mode; see `Command::NormalMode`)
+- [ ] Search: `/`, `?`, `n`, `N`
+- [ ] Pending-argument capture in the keymap, then `f` `t` `F` `T` and `r`
+- [ ] Honour the `:w <path>` argument instead of discarding it
+      (`run_command_line` binds it as `_arg`)
+- [ ] Prompt on `:q` with unsaved changes rather than only refusing
+
+### 2 — Editor features
+
+- [ ] Multiple buffers and `:b` / `gn` / `gp`
+- [ ] Auto-indent on newline; `>` and `<` to shift lines
+- [ ] `J` join lines, `~` switch case, `.` repeat
+- [ ] Match mode `m` — matching bracket, surround
+- [ ] Soft wrap (reworks the viewport anchor — see `MJB-DR-003`)
+- [ ] Multiple cursors (`Selection` is already shaped for it — see `MJB-DR-005`)
+
+### 3 — Language support
+
+- [ ] tree-sitter syntax highlighting
+- [ ] `languages.toml`
+- [ ] LSP client: diagnostics, completion, goto-definition
+
+### 4 — Project and process
+
+- [ ] CI: build, clippy, test, `scripts/check-trace.sh`, coverage
+- [ ] Resolve the six open derived requirements in
+      `docs/requirements/derived.md` — each needs a judgement, not code
+- [ ] Decide on ropey: stay on `2.0.0-beta.1` or move to stable `1.6.1`
+      (see `docs/reviews/library-selection.md`)
+- [ ] Decide whether to publish to crates.io at all, or self-host only
+- [ ] Replace the `unwrap` calls in `src/tui.rs::Drop` inherited from the
+      application template
+
+## Development
+
+```sh
+cargo build
+cargo clippy --all-targets -- -D warnings
+cargo test
+./scripts/check-trace.sh
+cargo llvm-cov --summary-only test
+```
+
+`direnv` users get `.envrc`, which points config and data at the working tree.
+
+### Architecture
+
+- `src/buffer/` — the editor core: rope document, selection, transactions,
+  motions, viewport, keymap. No `ratatui` dependency, so it is testable without
+  a terminal.
+- `src/components/buffer.rs` — the only widget; renders the visible window.
+- `src/app.rs` — event loop and global key routing.
+
+Text is held in a [ropey](https://github.com/cessen/ropey) rope and addressed by
+**byte** offset throughout. Rendering is O(viewport): the view is anchored by
+the byte offset of the first visible line, and only that window is walked, so
+per-frame cost does not scale with file size.
+
+## Process
+
+Developed to **DO-178C DAL-C**. Start with **[`docs/process.md`](docs/process.md)**,
+which explains the requirement artifacts, the ID system, and what a commit must
+satisfy.
+
+- `docs/process.md` — the system, and the commit rules
+- `docs/requirements/` — high-level, low-level, and derived requirements
+- `docs/traceability/trace.md` — HLR → LLR → source → test matrix
+- `docs/reviews/` — review checklists and library-selection rationale
+
+Every low-level requirement is tagged in source as `// MJB-LLR-nnn` and has a
+test named `mjb_llr_nnn_*`. The matrix is checked mechanically, not by
+inspection:
+
+```sh
+./scripts/check-trace.sh
+```
+
+It verifies traceability in **both** directions — requirements with no
+implementation or no test, and tests naming a requirement that no longer
+exists. All three sets must be empty. Run it before every commit.
+
+`git config commit.template .gitmessage` sets up the commit format; the template
+lists the change classes and the pre-commit gate.
+
+Current: 294 tests, clippy clean at `-D warnings`, 98/98 requirements traced,
+96.2% statement coverage of `src/buffer/**`.
+
+## License
+
+BSD 2-Clause. See [LICENSE](LICENSE).