max / quasi

Hold two markups at one position, as Op::Arms A guard derives as a branch only when turning it off deletes bytes. A renderer that draws one thing when a value is present and a different thing when it is absent deletes nothing, so there was no gap to measure and no branch to make -- the case the derivation started refusing in f2d4dd9. The pager's numbered strip is the site it was refused on: the page a reader is on is a readout where every other page is a control, deliberately, because a control that reloads the page it is on answers nothing. `swapped` makes `shrunk`'s measurement and keeps both middles instead of one. Both ends are pushed out past any sentinel they landed inside, since a maximal common prefix runs into the middle of a hole whenever two renders put the same one in the same place, and half a sentinel either side of an arm is `ZQH` reaching a reader. Two separate differences become one wide swap, unchanged middle and all, which rebuilds either render exactly; what it costs is room, so `build` refuses a swap another site nests inside rather than dropping structure the unrendered arm never showed. The arms' holes are numbered in the level around them, not in an arm, which is what lets one arm carry a hole the other does not: the strip's readout has a page number and no address, its control has both. That falls out of holes being filled by number rather than by position. One call site in the generated filler for both shapes, because the macro cannot know which the derivation will produce -- the fill program is written from the declaration and the residual is derived by rendering it. `Cursor::choose` runs the guarded item's own program on a branch and walks the chosen arm on arms. /feed still does not derive, and no longer for a staging reason: it shares its shapes with the library panel through a `Surface` argument, and a staged `include` drops its arguments. quasicoherent cbb63155.