A PUT that omits sections (an iOS tiles-only reorder) reset the user's
section customization to defaults, and symmetrically a sections-only PUT
reset the stored tiles. serializeInsightsLayout now takes the resolved
stored layout and falls back to it — not to defaults — for whichever
dimension the body omits; the PUT handler loads the stored layout first.
A first-ever single-dimension PUT (no stored layout) still fills
defaults, so the v2 blob stays complete. Pins the merge behaviour with
three unit cases.