slosh: fold floating panes and the drop grammar into DESIGN.md

D22 records the float contract: the minimized pattern (a flag and a wanted
rect on a leaf that keeps its seat), derived-vs-remembered rects, the hit
list as the z-order, the pop to the centre, the shadow-not-a-state, the
backdrop invariant, edge-pinned resizes, and the key decisions -- with M10c
(layout files, dump-layout, placement over the API, graphics occlusion)
recorded as still owed. M11 extends the drag machine section with the drop
grammar: swap centre plus third-width insert bands, the fill as the
promise, release derived from the painted hits, and the sibling-reorder
rule. The working plan file served its purpose and goes; git remembers it.

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