chore: establish rustfmt baseline

.github/workflows/pull-request.yml

@@ -52,4 +52,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
       - uses: swatinem/rust-cache@v2
-      - run: cargo check --all-targets
+      - name: Check formatting
+        run: cargo fmt --all -- --check
+      - name: Check compilation
+        run: cargo check --all-targets

TODO.md

@@ -8,6 +8,7 @@ Snapshot: 2026-07-28.
 
 ### What works
 
+- `cargo fmt --all -- --check` passes and is enforced for pull requests.
 - `cargo check --all-targets` passes.
 - All 47 integration specifications in `tests/language_specs.rs` pass.
 - The crate exposes checked file/source execution APIs and typed I/O, parse, and runtime error categories.
@@ -16,7 +17,6 @@ Snapshot: 2026-07-28.
 ### Quality gaps
 
 - `cargo test` fails because the library doctest uses invalid syntax and nonexistent `Value::Integer`.
-- `cargo fmt --all -- --check` reports an import-order difference in `src/errors.rs`.
 - Strict Clippy reports 36 errors across the interpreter, lexer, parser, type inference, and symbol table.
 - Panic recovery wraps rather than removes many panic, `unwrap`, and `expect` paths; the interpreter alone contains roughly 90.
 - `parser.rs` and `interpreter.rs` are approximately 975 and 1,217 lines and mix several responsibilities.
@@ -70,7 +70,7 @@ Snapshot: 2026-07-28.
 
 ## Phase 1 — Clean Legacy Baselines
 
-- [ ] Open and complete an issue that applies rustfmt, adds `cargo fmt --all -- --check` to CI, and changes no behavior.
+- [x] Open and complete an issue that applies rustfmt, adds `cargo fmt --all -- --check` to CI, and changes no behavior.
 - [ ] Fix the library doctest against the v0.1 API, make `cargo test` green, and require it in CI.
 - [ ] Resolve strict Clippy findings without suppressing project-wide lints, then require `-D warnings` in CI.
 - [ ] Separate generated mdBook output from sources and define one reproducible documentation build command.

src/errors.rs

@@ -1,7 +1,7 @@
+use std::cell::Cell;
 use std::error::Error;
 use std::fmt;
-use std::cell::Cell;
-use std::panic::{catch_unwind, set_hook, take_hook, UnwindSafe};
+use std::panic::{UnwindSafe, catch_unwind, set_hook, take_hook};
 use std::sync::{Mutex, OnceLock};
 
 #[derive(Debug, Clone, PartialEq, Eq)]