perf(web): isolate Excel export worker

vehicle-data-platform/docs/frontend-production-readiness.md

@@ -90,6 +90,8 @@ The first continuity pass only warmed Monitor, Tracks, History and Statistics in
 
 A production route-memory loop did not prove a retained application leak: after natural collection, JS heap returned to roughly its initial 31 MB range and old map documents/frames were released. It did expose an ineffective rendering hint: `content-visibility: auto` was attached to the always-visible vehicle scroll viewport instead of the 200 independently off-screen rows. The containment boundary now lives on every desktop rail row and mobile list card, paired with `contain-intrinsic-size` so skipped content keeps stable scroll geometry. This follows the CSS Containment specification's long-scroll-list use case while avoiding the documented scrollbar jump caused by missing intrinsic size: <https://www.w3.org/TR/css-contain-2/#content-visibility>.
 
+Excel export previously produced two independent 940 KB ExcelJS assets because the worker imported the browser download module while that module also retained a main-thread workbook fallback. Workbook construction now lives in a worker-only runtime module; unsupported legacy browsers receive an upgrade message instead of freezing the page with a main-thread build. The production output contains one ExcelJS asset, the Statistics route chunk is smaller, and `scripts/verify-build.mjs` makes exactly one ExcelJS asset plus one mileage worker a mandatory `pnpm build` gate.
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 The navigation and progressive-loading direction follows mature observability systems: persistent global controls, collapsible sections and loading only the content needed for the current task. Elastic documents these dashboard interaction and panel-organization patterns here: <https://www.elastic.co/docs/explore-analyze/dashboards/using> and <https://www.elastic.co/docs/explore-analyze/dashboards/arrange-panels>.
 
 ### Remaining audit queue