test: split unit tests into node:test suite, add c8 coverage

[robertsLando]

Summary

Phases 1 + 2 of #267. Phase 3 (migrating e2e onto node:test) intentionally dropped — the custom test/test.js harness is battle-tested across 169 e2e directories and the maintainer prefers to keep it.

• Phase 1 — test/unit/*.test.ts on Node's built-in node:test, loaded via esbuild-register so lib sources are imported directly (no yarn build). Three in-process tests migrated out of the e2e harness:
  • test-00-sea-picker/ → test/unit/sea-picker.test.ts
  • test-48-common/ → test/unit/common.test.ts (platform-split via describe(..., { skip }))
  • test-50-config-parse/ → test/unit/config-parse.test.ts (ad-hoc t() harness replaced with node:test + beforeEach/afterEach)
  • Old dirs deleted; test-00-sea-picker removed from the npmTests list in test/test.js.
  • New scripts: yarn test:unit, yarn test:unit:watch.
• Phase 1.5 (extended coverage) — now that the suite exists, filled in the pure-pure modules identified as priorities in the issue:
  • compress-type.test.ts — CompressType enum stability + Zstd accessor branches
  • detector.test.ts — parse() tolerant flags, detect() warn-don't-throw, every public visitor
  • esm-transformer.test.ts — rewriteMjsRequirePaths across quote styles, transformESMtoCJS for top-level-await wrapping (with/without exports) and import.meta shim injection
  • common.test.ts extended: isPackageJson / isDot{JS,JSON,NODE} / unlikelyJavascript / replaceSlashes / isRootPath / isESMPackage / isESMFile (last two use an mkdtemp fixture)
  • config-parse.test.ts extended: isConfiguration, stringifyTarget, PKGRC_FILENAMES precedence, findPkgrc (mkdtemp fixture)
  • 162 assertions run in ~1.7s (up from 96 in ~1.3s).
• Phase 2 — c8 (added as --exact devDep) wraps NODE_V8_COVERAGE; the e2e path inherits the env var through utils.pkg.sync and captures subprocess coverage without code changes. Scripts: yarn coverage:unit, yarn coverage:e2e (--clean=false), yarn coverage for the merged report. Excludes wired in .c8rc.json.
• CI — yarn test:unit now runs in the existing build matrix (3 OS × Node 22/24) so common.test.ts's win32 branch and Node 24 get coverage at ~1s/cell.
• Tooling — tsconfig.test.json extends the base for typescript-eslint's parser to see test/unit/**/*; eslint.config.js's parserOptions.project is now an array.
• Integration-heavy modules (walker, producer, packer, fabricator, resolver, sea-assets, refiner, follow, mach-o, chmod, bin, help, colors, index, most of sea) intentionally not unit-tested — they inherently need fixtures / subprocesses / binary manipulation that the e2e suite already exercises end-to-end.

Coverage baseline (unit only)

From yarn coverage:unit:

File	Coverage
lib/ overall stmts	35.62%
lib/ overall branch	79.78%
common.ts	79.20%
compress_type.ts	77.27%
config.ts	67.69%
detector.ts	80.53%
esm-transformer.ts	74.88%
options.ts	78.26%
sea.ts	25.86% (pickMatchingHostTargetIndex)

Other lib/*.ts files are exercised only by e2e today; the merged yarn coverage report is the right way to see total coverage.

Test plan

• yarn test:unit — 162 pass, 0 fail, ~1.7s
• yarn coverage:unit — lcov + text reporters emit, baseline captured
• yarn lint — clean
• npx tsc -p tsconfig.test.json --noEmit — clean
• Spot-run one e2e test (test-50-api) to confirm test/test.js harness still works after npmTests edit
• First CI run (96-test version) — all 27 checks green on Linux / macOS / Windows × Node 22 / 24
• CI green on extended suite

Closes #267.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Introduces a fast unit-test suite alongside the existing e2e harness, and adds V8-based coverage reporting to improve feedback speed and visibility for lib/ behavior.

Changes:

• Adds node:test-based unit tests in test/unit/*.test.ts, migrating three prior in-process “e2e” tests into the unit suite.
• Adds c8 coverage tooling (+ config) and new yarn scripts for unit/e2e coverage reporting.
• Updates CI/docs/tooling configs to recognize and run the new unit suite.

Reviewed changes

Copilot reviewed 16 out of 18 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
yarn.lock	Locks new dev dependency graph for c8 and its transitive deps.
tsconfig.test.json	Adds a TS config for typechecking unit tests alongside lib/**.
test/unit/sea-picker.test.ts	New unit coverage for SEA host/target matching helper.
test/unit/config-parse.test.ts	Migrates config parsing/flag resolution validation to node:test.
test/unit/common.test.ts	Migrates lib/common path/snapshot helpers test with platform-skipped suites.
test/test.js	Removes the migrated test-00-sea-picker directory from the e2e npm test list.
test/test-50-config-parse/main.js	Deletes old in-process config parser test previously living in e2e structure.
test/test-48-common/main.js	Deletes old in-process common test previously living in e2e structure.
test/test-00-sea-picker/main.js	Deletes old in-process SEA picker test previously living in e2e structure.
package.json	Adds c8 devDep and introduces test:unit + coverage scripts; wires unit tests into yarn test.
eslint.config.js	Updates TS parser config to include both tsconfig.json and tsconfig.test.json.
docs-site/development.md	Documents the new unit suite and coverage workflows.
CLAUDE.md	Updates quick reference to mention yarn test:unit and clarify build requirement is for e2e.
.gitignore	Ignores coverage/ outputs.
.github/workflows/ci.yml	Runs yarn test:unit in the existing OS/Node build matrix.
.github/copilot-instructions.md	Updates contributor instructions to describe unit suite + coverage commands.
.claude/rules/testing.md	Updates internal testing guidance to include the unit suite and coverage commands.
.c8rc.json	Adds c8 include/exclude/reporting configuration for lib/**.

[Copilot]

Pull request overview

Copilot reviewed 20 out of 22 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 23 out of 25 changed files in this pull request and generated 2 comments.