test: cover Node-specific paths and namespace matching

[appsir2016-lab]

Adds 31 tests to the two existing test files. No source changes, no new dependencies, and the test:node script is unchanged. The tests added to test.js use no Node APIs; the karma suite was run locally with headless Chrome and passes (22 tests, 8 of them new).

Statement coverage of npm run test:node, measured with npx c8 --all --src src --exclude 'test*.js':

File	Before	After
src/node.js	58.2%	100%
src/index.js	90.0%	100%
src/common.js	94.9%	99.3%
Node-side files together	77.7%	99.7%

What is covered now:

• The extended palette when supports-color reports level 2+, the basic palette otherwise, and the older module shape without stderr. Four tests intercept Module._load for supports-color so the branch runs regardless of whether the optional module is installed on the machine.
• DEBUG_* environment parsing and value coercion.
• useColors() with inspectOpts.colors set and with the TTY fallback.
• formatArgs: colour prefix, the 256-colour escape, multi-line prefixes, the date prefix and hideDate.
• The %o and %O formatters, including per-instance inspectOpts and colours passed to util.inspect.
• save, load, log, per-instance inspectOpts from init, and the deprecated destroy() no-op.
• The Electron/browser detection in src/index.js.
• %%, a non-string first argument, Error coercion with and without a stack, wildcard backtracking and trailing wildcards, skip lists, and whitespace-separated namespaces.

Two things noticed while running the suite and left alone here:

• xo 0.23 crashes on current Node (util.isDate is not a function, from core-assert via eslint-plugin-ava), so npm run lint needs Node 22 or lower, or a newer xo. I ran the linter on the two test files with a local shim restoring the removed util.is* helpers for the linter process only; both files are clean.
• destroy() in src/common.js is unreachable in the Node build because src/node.js supplies its own, so its two lines stay uncovered.

Checks: npm run test:node passes with 47 tests on Node 26, and npm run test:browser passes with 22 tests in headless Chrome 152.