Task 4 of v0.3 plan.
useLiveValues(filePath, pouName, intervalMs=500) sets up a recursive
setTimeout poll. Returns the values map iff readLiveValues returns
ok AND the snapshot's pou_name matches the requested pouName;
returns null otherwise (cursor not on a POU, file missing/stale,
wrong POU snapshot).
Cancelled flag + setTimeout (not setInterval) so the cleanup is
race-free: if the file read takes longer than intervalMs, the next
tick won't pile up.
Browser computes the live-values file path once via
liveValuesFilePath() (catches Windows LOCALAPPDATA-missing -> null
so we don't crash; pump just won't be able to write either) and
passes the resulting map down to <Viewer liveValues={...}/>.
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