Replace the 105-link npm test chain with a discovering, parallel test runner

[domidex01]

The problem

npm test was a single && chain of 105 npm run links — a six-thousand-character line. It spawned a fresh npm per file, ran strictly serially, and short-circuited at the first failure: on Windows it dies at link 15 (test:popoverdropdown) and the other ninety tests never run at all. Adding a test file meant hand-editing that line in addition to adding the per-file script — and at least one test (test/doc-defaults.js) existed but had never been added to the chain, so the gate silently never ran it.

The change

A small scripts/test.js that:

• discovers test/*.js, so a new test file is in the gate the moment it exists — doc-defaults.js is now running (and passing);
• runs files in parallel with a concurrency cap (--jobs=N, default min(8, cores)) — safe because every test uses listen(0) and mkdtemp, verified by comparing parallel results against isolated serial runs;
• prints every failure rather than the first, with each failing file's full output, and exits non-zero if any failed;
• routes *.test.js through node --test, thumbs.js through electron, and everything else through plain node (with --disable-warning=MODULE_TYPELESS_PACKAGE_JSON, which two scripts already used);
• takes filename substrings as arguments to run a subset (node scripts/test.js focus color).

The 105 per-file test:* scripts stay for running one test at a time.

Numbers

Full suite on Windows: 45 seconds wall-clock, 110 files, 100 passed, 10 failed — where the old chain died at file 15 and reported one failure. The 10 failures are pre-existing: each fails identically when run alone, serially (code-search, field-focus, git-snapshot, git-branches, panel-field, popover-dropdown, preview-worktree, starter, section-dot, vars-row-height). The chain was hiding all but the first of them.