Route the page mount through onStateChange: renderXXX does routing +
declareGadget + child.render (the fetch), holds the live child instance
in props.main_gadget (DOM-bearing, not JSON), then forces the mount via a
fresh-seq main_content state key. onStateChange's main_content branch does
getElement + clear + append (editor.js precedent). changeState resolving
after the mount preserves the original ordering (decreaseLoadingCounter
still runs after paint).

Error path (displayErrorContent) stays imperative as the documented
exception: it runs after MAIN_SCOPE is dropped, outside the render
lifecycle, faithful to upstream rjs_gadget_erp5_js.js (same precedent as
gadget_interface.js).

New-code assertion: the in-app JSLint row was NOT clean before (4 undefined
globals — URI, Uint8Array, Blob, FileReader); declared all four; now 0
errors. Live: deploy verify OK, smoke 22/22, QUnit 113/113, 0 console
errors. This was the last gadget — every gadget now writes DOM only in
onStateChange (bar the two documented upstream-faithful exceptions).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>