chore: update CI matrix to Node 20, 22, 24 — drop EOL Node 18

Node 18 reached EOL April 2025. Node 24 is the current LTS target.

Changes:
- CI matrix: [18, 20, 22] → [20, 22, 24]
- package.json engines: >=16.7.0 → >=20.0.0
- Removed Node 18 conditional in CI (c8 coverage works on all 20+)
- Simplified CI to single test:coverage step for all versions

797/797 tests pass on Node 24.

.github/workflows/test.yml

@@ -22,7 +22,7 @@ jobs:
       fail-fast: true
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        node-version: [18, 20, 22]
+        node-version: [20, 22, 24]
 
     steps:
       - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
@@ -37,13 +37,5 @@ jobs:
         run: npm ci
 
       - name: Run tests with coverage
-        # c8 v11 requires Node 20+ (engines: ^20.0.0 || >=22.0.0). Node 18 EOL April 2025.
-        # Use bash on all platforms so shell glob expansion works on Windows.
-        if: matrix.node-version != 18
         shell: bash
         run: npm run test:coverage
-
-      - name: Run tests (Node 18, coverage not supported)
-        if: matrix.node-version == 18
-        shell: bash
-        run: npm test