fix(colitur-x): make the selection and the I-class marker legible

Both markers were drawn in gold, which works on a dark cell and very
nearly disappears on a white one -- and most days are white.

The selection is now ringed twice, dark outside and bright inside, so it
reads against a white cell and a red one without knowing which it is. The
I-class marker is a badge rather than coloured text: filled with the
cell's own foreground and lettered in its background, so it inverts
against whatever the day's liturgical colour is.

It also opens on TODAY when the input covers it, which is what a viewer
should do; colitur's kernel still never reads a clock.

Drawing now goes to an off-screen pixmap and CX_DUMP writes one frame
out, so the result can be checked without screenshotting a window whose
contents X does not retain while it is obscured.

1 files changed, 7 insertions, 0 deletions

diff --git a/README.md b/README.md
index 9df7b8e..6530ec9 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,13 @@ liturgical colour, and shows the selected day's readings underneath.
 
 Keys: arrows or `hjkl` move, `n`/`p` change month, `q` quits.
 
+It opens on **today** when the input covers it. Reading the clock is a
+presentation choice and belongs in a viewer: colitur's own kernel is pure
+and deliberately never asks what day it is, which is what lets it be tested
+to the year 9999.
+
+    CX_DUMP=/tmp/f.ppm colitur-x < day.csv   # write one frame, then exit
+
 It reads CSV on **stdin** and nothing else, so the shell decides what it
 shows. One month: