docs: the backdrop feed measures window content, not just the desktop

cce-fx@00ff81c reads an occluding window's pixels rather than reporting
it as unknown, so `unknown` is now the narrow case (no committed buffer,
unsupported read format) instead of "anything is in the way".

Co-Authored-By: Claude Opus 5 <[email protected]>

 CLAUDE.md | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/CLAUDE.md b/CLAUDE.md
index b707286..bb43508 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -204,10 +204,14 @@ closes that loop with the compositor, which CAN see:
 3. `tick` eases `halo_now` toward that demand over ~120ms. Stepping straight
    to it makes the outline strobe as the desktop pans under the segment.
 
-Two failure directions, both deliberately resolved toward legible: an
-unparseable or absent line reads as `(50, 100)` — "unknown, assume the worst"
-— and so does a window overlapping the segment, whose pixels the compositor
-cannot know either.
+A window covering part of a segment is measured too — the compositor reads
+that window's own content over the overlapping strip and blends it with the
+desktop reading for the rest. `unknown` is left for content it genuinely
+cannot read (no committed buffer, an unsupported read format).
+
+Failures resolve toward legible in every direction: an unparseable or absent
+line reads as `(50, 100)` — mid luminance, full spread — which drives the
+outline rather than switching it off.
 
 ## Interactions worth knowing before touching input code