After m/paliad#69's edit-mode overhaul, widgets visually overlapped on
mixed-size rows: a 12-col + 6-col swap, an auto-flow widget landing on
an explicit blocker, or a resize-grow into a sibling all produced
layouts that ignored colspan footprints when computing occupancy.
Extracts placement math from dashboard.ts into a pure ./dashboard-grid
module and adds an occupancy bitmap. Every visible widget is placed
once; explicit-position collisions are resolved by searching downward
from the requested row for the first w×h block that fits, preferring
the requested column. Resize-grow + drag-drop swap now reliably
produce no-overlap layouts because the placer cleans up after them.
x+w > GRID_COLUMNS is clamped in the placer instead of rendered as an
overflow — matches the validator's hard rule on the wire.
Adds 14 dashboard-grid.test.ts regressions covering the mixed-width
swap, resize-grow shifting siblings, multi-row widgets, and the
overflow clamp. Pure tests — no DOM.
mAi
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