require editor margins to be ints

Dependencies

• [2] UHB4GARJ left/right margin -> left/right coordinates
• [3] 2L5MEZV3 experiment: new edit namespace
• [*] BJ5X5O4A let's prevent the text cursor from ever getting on a drawing

Change contents

• replacement in edit.lua at line 93
  [2.22362]→[2.22362:22398](∅→∅)
  − left = left,

  − right = right,
  [2.22362]

  [2.22398]
  + left = math.floor(left),

  + right = math.floor(right),
• edit in Manual_tests.md at line 49
  [3.16090]

  [3.16090]
  + * I wish I could require pixel coordinates to integers. The editor defensively

  + converts input margins to integers.

  +