test: migrate from jest to node:test runner

[arb]

Tracks #260. First step in evaluating a move off Jest, prompted by Jest's slowing release cadence. This PR keeps the framework decision narrow: swap the harness with the smallest possible test-body churn, document the tradeoffs, and let the rest of #260 weigh whether to stick with node:test or revisit Vitest as a follow-up.

What changed

• Runner: node --test (Node's built-in test runner; no --experimental-vm-modules flag needed anymore).
• Assertions: the standalone expect package — the same code Jest ships internally — so the ~240 existing expect(...) call sites stay verbatim.
• Mocks: jest.fn() → mock.fn() from node:test. The 10 .toHaveBeenCalled() sites became .mock.callCount() checks because the standalone expect matchers expect Jest-mock shape that node:test mocks don't provide.
• Parameterization: 4 describe.each template-literal tables rewritten as plain for...of loops.
• Snapshots: 4 toMatchSnapshot() sites inlined as literal expect(...).toEqual(...) assertions. The Map snapshot was rewritten as targeted .message assertions instead of a structural equality check, because joi.ValidationError instances carry internal state that makes deep equality fragile.
• is-ci-cli is preserved — the local-watch / CI-one-shot UX is unchanged.

Coverage tooling (key #260 data point)

node --test doesn't enforce coverage thresholds and the existing coverage job in .github/workflows/ci.yml consumes ./coverage/lcov.info for Codecov. To preserve both today's 100% threshold gate and the lcov upload, this PR adds c8 as a devDep and runs c8 --reporter=lcov --reporter=text --100 node --test test/ under test:ci.

This is the single biggest gap between node:test and Jest for this repo: Jest had threshold enforcement built in; node:test requires a separate tool (c8) or a custom parser to get the same gate. Worth weighing in #260 against alternatives like Vitest, which bundles coverage + thresholds natively.

Net dep change

• Removed: jest (and its tree)
• Added: expect@30.x.x, c8@10.x.x
• node_modules net change: -178 packages, +6 packages

Support matrix

Unchanged. engines: ">=20" stays, .github/workflows/ci.yml matrix (Node 20.x/22.x/24.x/25.x × Express 4/5) stays. Verified locally on Node 20.19 (matrix floor): 62 tests pass, 100% coverage, coverage/lcov.info produced, eslint clean.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b36068f) to head (7dad645).
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Additional details and impacted files

@@            Coverage Diff             @@
##            master      #265    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files            4         5     +1     
  Lines          100       263   +163     
  Branches        14         0    -14     
==========================================
+ Hits           100       263   +163     

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[cjihrig]

@arb is my hero.

By the way, node:test does support snapshots and code coverage (it's listed as experimental, but that is mostly just in name at this point).