Reconcile the shared HTTP handlers and discovery rules from the split PRs with the complete fixture surface in wpt-misc-fix. Share stash namespaces and XHR dispatch, retain the existing testdriver bridge, preserve raw header values, and carry forward the remaining pipe, template, and dynamic-import regressions.
Validation: 742 Python tests across 74 modules pass; focused Ruff checks and git diff --check pass. With fixed Moli and Chromium binaries, a 27-case WPT comparison against main improves whole-case passes from 7 to 26 and 6 to 23 respectively, with no previously passing case regressing.
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