Add node-qunit coverage for formats/JitGraph/SRF_JitGraph.js (0%, 59 lines)

[gesinn-it-gea]

Follow-up to #1098. formats/JitGraph/SRF_JitGraph.js currently has 0% node-qunit coverage (59 statement lines), now visible thanks to the nyc include-glob fix in #1095.

Important: this file has never been covered by tests, so it cannot be assumed the code is currently bug-free. Do not write tests that simply assert today's behavior — verify each assertion against the code's actual logic (and the SRF documentation where applicable). If you find suspicious or clearly incorrect behavior while writing tests, report it as a bug (or file a separate issue) rather than writing a test that locks it in as "correct".

Steps

1. Add a node-qunit test file under tests/node-qunit/ exercising formats/JitGraph/SRF_JitGraph.js's public entry points (mirror the require()-and-invoke pattern used by existing tests, e.g. ext.srf.formats.gallery.*.test.js or ext.srf.formats.tagcloud.test.js).
2. Run npm run test-coverage (or make ci-coverage) and confirm coverage for this file increased from 0%.
3. Run npm run node-qunit to confirm no regressions in the rest of the suite.