fix(playback): keep CSS bounds unrounded until native scaling

Review feedback on #1206: measureBounds() rounded the CSS edges in the
renderer, before the main-process CSS-to-native conversion, so fractional
layout positions could drift by a pixel per scale factor (a 10.49px edge
at 200% must land on 21 physical px, not 20). The renderer now sends raw
getBoundingClientRect() edges and rounding happens exactly once, after
scaling. Also pins process.platform explicitly in the macOS wiring test
instead of relying on the suite default.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/architecture/embedded-mpv-native.md

@@ -607,9 +607,11 @@ and match physical pixels only at 100% page zoom AND 100% display scale.
 `EmbeddedMpvNativeService` therefore converts every native-view bounds payload
 in the main process (`toNativeViewBounds` in `embedded-mpv-bounds.util.ts`):
 all platforms scale by the webContents zoom factor, win32/linux additionally
-by the scale factor of the display hosting the window. Edges are scaled
-before deriving width/height so rounding cannot open 1px seams against the
-surrounding DOM UI. Skipping this conversion is issue #1145: on scaled
+by the scale factor of the display hosting the window. The renderer sends
+unrounded CSS edges (`measureBounds` does not round) and the conversion
+rounds exactly once, after scaling — edges first, width/height derived from
+them — so fractional CSS layouts and fractional scales cannot open 1px
+seams against the surrounding DOM UI. Skipping this conversion is issue #1145: on scaled
 displays (Windows 125%, Linux fractional scaling, HiDPI TVs) the video landed
 toward the window's top-left corner at `1/scale` of its size, in windowed and
 fullscreen mode alike.