Fix CLI contract: distinct exit codes, version from package.json, Node 18 test runner - #3

Why this is core

A CLI's exit codes and version output are contracts — scripts and CI wrap corecoder-ts and depend on them. The TS port silently regressed three of them against the Python original and its own engines claim:

1. Error vs interrupt exit codes were collapsed — the Python original exits 1 on error and 130 on ^C; the TS port returned 130 for both. Any wrapper that checks $? can't tell a crash from a user cancel.
2. Version was hardcoded — src/cli.ts shipped const VERSION = '0.1.0' while package.json is the real source of truth; every release bump risks --version drifting.
3. npm test breaks on the declared Node floor — node --test dist/tests/*.test.js relies on the test runner expanding globs, which only landed in Node 21. On Node 18/20 — which engines claims to support — and on Windows cmd (which never expands), the literal pattern is passed through and the run fails.

Changes

Distinct exit codes (src/cli.ts)

• runTurn now reports whether a null result was a cancellation (aborted) or a failure.
• New exported turnExitCode(): 0 success, 130 ^C interrupt, 1 error — matching the Python original (KeyboardInterrupt -> 130, Exception -> 1) and POSIX convention (128 + SIGINT).

Version from package.json (src/cli.ts)

• VERSION (now exported) is read at runtime via createRequire relative to the module file — works both in the repo (dist/src/cli.js -> ../../package.json) and in a published tarball (same layout). A unit test pins it to package.json so drift fails CI.

Cross-version test runner (scripts/run-tests.mjs, package.json)

• New small runner resolves dist/tests/*.test.js itself and passes explicit paths to node --test — identical behavior on Node 18–24 and every shell, and new test files are picked up automatically (no script edits on adding a test).

Tests (tests/cli.test.ts, 2 new)

• turnExitCode: 0 / 130 / 1 matrix
• VERSION equals package.json version

All 26 tests pass (npm test via the new runner); --version prints corecoder-ts 0.1.0; demo exits 0.