[P3] junitParser parses every JUnit XML twice per module

[eboudrant]

Priority: P3 · Review finding: #18

src/scanner.js processSingleModule calls parseJunitXml (parses each TEST-*.xml for stats) and then parseDiffPercentages (re-reads and re-parses the same files for failure messages). Every XML is read + XML-parsed twice per module per scan/watch tick.

Fix: single pass in src/junitParser.js returning { stats, mtime, diffPcts }.