docs: note that sun_t passes a NaN timestamp through

NaN is a number, so the type guard admits it, and every comparison
against NaN is false, so neither clamp catches it and the function
returns NaN.

Left unguarded on purpose. No caller can produce it: parse() matches
%d+ for the sun times and `now` is os.time(). If one ever did, Cairo
ignores a NaN coordinate, so the arc would lose its dot rather than the
dashboard going blank. A ponytail: comment names the ceiling and where
the guard would go.

Found while probing the committed code, not by the tests, which do not
cover NaN.

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

1 files changed, 5 insertions, 0 deletions

diff --git a/lib/weather.lua b/lib/weather.lua
index f8c7517..84f355a 100644
--- a/lib/weather.lua
+++ b/lib/weather.lua
@@ -124,6 +124,11 @@ function M.sun_t(now, sunrise, sunset)
      or type(sunset) ~= 'number' then return nil end
   local span = sunset - sunrise
   if span <= 0 then return 0 end   -- polar day/night or bad data: no division
+  -- ponytail: a NaN timestamp slips through, since NaN is a number and every
+  -- comparison against it is false, so both clamps below fall through and this
+  -- returns NaN. Unreachable today: parse() matches %d+ for the sun times and
+  -- `now` is os.time(). Cairo ignores a NaN coordinate, so the cost would be a
+  -- missing dot, not a crash. Guard it here if a caller ever computes `now`.
   local t = (now - sunrise) / span
   if t < 0 then return 0 elseif t > 1 then return 1 end
   return t