jawstree: run production regression pages under jsdom

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Why

The two jawstree production regression tests launch headless Firefox to render a page and screenshot it. On macOS this intermittently aborts at Firefox startup — _RegisterApplication(), unable to get application ASN from launchservicesd (SIGABRT in TransformProcessType) — which fails the test run and pops a "Firefox has quit unexpectedly" crash dialog. The failure is environmental (GUI-session registration can be refused for processes launched from sandboxed/unattended contexts), so no browser-based harness avoids it.

What

• New jawstree/jsdom_test.go harness: loads a generated page under jsdom via node -e (runScripts: "dangerously", resources: "usable", pretendToBeVisual for the widget's requestAnimationFrame use) and passes only when the page turns its #result element green. On failure it reports the element state and any jsdom errors instead of an opaque red pixel.
• Both regression tests keep their pages and assertions byte-for-byte, only the loader changes; the screenshot/PNG-pixel readout is gone. No GUI process is launched, so the launchservicesd failure mode cannot occur on any platform.
• jsdom resolves from an ancestor node_modules (e.g. npm install jsdom at the repo root) or from the npm global root via NODE_PATH. Tests skip when node or jsdom is unavailable and fail instead when JAWS_REQUIRE_NODE is set, matching the existing node-driven test convention.
• GitHub runner images ship node/npm but no npm packages, so the build workflow installs jsdom (npm install --no-save jsdom) before the test step; .gitignore covers local npm artifacts. The 386 leg intentionally skips these tests (no jsdom there).

Verification

• Without jsdom: both tests skip; with JAWS_REQUIRE_NODE=1 they fail loudly.
• With jsdom: both pass, including under -race; a deliberately never-green page makes the runner exit non-zero with diagnostics.
• Full local gate: go vet, gofmt -l, staticcheck, golangci-lint, gosec, go build, go test -race ./... all clean.