Without `precision` 500 coordinates of every size reopen bit for bit and a
second save writes the same bytes. Halves round away from zero, a closed
path keeps its closing anchor on its first one, a shape smaller than one
step collapses onto one point, coordinates at the edge of the canvas round
to the asked places, and saving a rounded file again at the same precision
writes the same bytes. docs/mcp.md states these rules.
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