ci: smoke test the build output on Node.js 18

[shaurya703]

Closes #57

engines.node is ">=18" but the CI matrix starts at 20, so the oldest supported version was never exercised. #109 tried adding 18 to the existing matrix and stalled: tsdown, vitest and even node:test are unusable on 18.x, and downgrading the dev stack to reach it was not worth it.

This takes the approach settled on in that thread instead:

• a new test case (or describe block) which tests dist/
• a standalone test file excluded from vitest which uses node:assert but no test runner

test/smoke.mjs checks the happy path of the built bundle:

• x() — stdout, stderr, argument forwarding, async iteration, exit codes, throwOnError, and env
• xSync() — stdout, exit code, throwOnError
• exec/execSync alias identity

Why this one works where #109 didn't

The job deliberately skips npm ci. The smoke test imports dist/ and node builtins and nothing else, so the dev dependencies that made 18.x unworkable are never installed. It runs on 18.0.0 and 18.x across ubuntu and windows, against the same dist/ artifact the other jobs already consume.

process.execPath is used for every child command, so the same file runs unmodified on Windows.

vitest only collects src/**/*_test.ts, so the file is excluded from the unit run without any extra configuration.

Verification

Run against a real v18.0.0 (not just 18.x): 10/10 pass.

The suite isn't vacuous — stubbing the exitCode getter in dist/ turns 3 of the 10 red and exits 1.

On the current toolchain, vitest still reports 51 passed, and lint, publint and format:check stay clean.