ci: drop Node 18 from test matrix — vitest 4/rolldown requires Node 20+

[dmchaledev]

Problem

main's current HEAD (0ce2274, the commit that added the Node 18/20/22 test matrix in #66) has a failing CI run (workflow run 27606677557, conclusion: failure). The Node 18 job fails before a single test executes:

node_modules/rolldown/dist/shared/rolldown-build-CrPk_lZe.mjs:9
import { formatWithOptions, styleText } from "node:util";
SyntaxError: The requested module 'node:util' does not provide an export named 'styleText'

Root cause: vitest@4.1.7's dependency chain (vite → rolldown) requires Node ^20.19.0 || >=22.12.0 (confirmed via npm install --include=dev's EBADENGINE warnings on Node 18). This is a test-tooling constraint, not a constraint on the published library itself (src/ only uses fetch/AbortController, both fine on Node 18).

I spot-checked several of the currently-open PRs (#98, #99, #103, and this repo's own PR history) and every one fails the same Node 18 leg for the same reason — it's not specific to any one PR's changes. This means CI has been red on main and on every open PR since 2026-06-16, which lines up with the merge freeze described in #102 ("no PR has merged into main since 2026-06-16").

Fix

Drop '18' from .github/workflows/ci.yml's test matrix, leaving ['20', '22']. package.json's engines field (>=18.0.0) is left untouched since it describes the published library's runtime requirement, not the test toolchain's.

Verified locally

• npm install --include=dev — clean (Node 22)
• npm run typecheck — passes
• npm test — 85/85 pass
• npm run build — passes

Related

• PR pipeline has stalled since 2026-06-16 — 56 open PRs, heavy duplication, nothing merging #102 — tracks the broader PR-backlog/merge-freeze problem this CI failure very likely contributes to (or fully explains, if the Node 18 check is a required status check).
• Not a duplicate of any currently open PR — none of the 57 open PRs address the CI matrix itself.

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