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, 36 insertions, 0 deletions

diff --git a/src/logging.rs b/src/logging.rs
new file mode 100644
index 0000000..fd4e0b6
--- /dev/null
+++ b/src/logging.rs
@@ -0,0 +1,36 @@
+use std::sync::LazyLock;
+
+use tracing_error::ErrorLayer;
+use tracing_subscriber::{EnvFilter, fmt, prelude::*};
+
+use crate::config;
+
+pub static LOG_ENV: LazyLock<String> =
+    LazyLock::new(|| format!("{}_LOG_LEVEL", config::PROJECT_NAME.clone()));
+pub static LOG_FILE: LazyLock<String> = LazyLock::new(|| format!("{}.log", config::APP_NAME));
+
+pub fn init() -> color_eyre::Result<()> {
+    let directory = config::get_data_dir();
+    std::fs::create_dir_all(directory.clone())?;
+    let log_path = directory.join(LOG_FILE.clone());
+    let log_file = std::fs::File::create(log_path)?;
+    let env_filter = EnvFilter::builder().with_default_directive(tracing::Level::INFO.into());
+    // If the `RUST_LOG` environment variable is set, use that as the default, otherwise use the
+    // value of the `LOG_ENV` environment variable. If the `LOG_ENV` environment variable contains
+    // errors, then this will return an error.
+    let env_filter = env_filter
+        .try_from_env()
+        .or_else(|_| env_filter.with_env_var(LOG_ENV.clone()).from_env())?;
+    let file_subscriber = fmt::layer()
+        .with_file(true)
+        .with_line_number(true)
+        .with_writer(log_file)
+        .with_target(false)
+        .with_ansi(false)
+        .with_filter(env_filter);
+    tracing_subscriber::registry()
+        .with(file_subscriber)
+        .with(ErrorLayer::default())
+        .try_init()?;
+    Ok(())
+}