Watermark.inset becomes inset_x (from the left or right edge) and inset_y
(from the top or bottom edge), like Lightroom Classic's watermark editor.
The watermark keys are insetX and insetY; the old inset key still reads as
both, so saved presets and existing MCP calls keep working.
The Export dialog's Inset slider becomes Horizontal inset and Vertical
inset, each greyed out when the position is centred along its axis.
cargo fmt reflows WM_POSITIONS and its test from the previous commits.
Co-Authored-By: Claude Opus 5.5 <[email protected]>
