Drop the timing and walking hint lines

Both restated their own chart in prose. The timing hint gave the typical pee gap
as a sentence directly under a row whose whole job is to show that gap on a
track, and the walking one gave a target the walk trend already draws as a line
with a chip naming the number.

Removed with them: the pee-gap cache in renderTiming, which existed only so the
hint and the pee row could share one computation, and the .timing-hint rule.
median and walkTargetFor stay — the timing chart and the walk trend's goal line
are still their callers.

The changelog entry announcing walks keeps its mention of the rule of thumb.
That one shipped and was true at the time, so a new entry retires it rather than
an edit pretending it was never there.

src/changelog.json

@@ -1,4 +1,5 @@
 [
+  { "date": "2026-09-01", "text": "Dropped two hint lines: the “Based on N pee gaps…” sentence under the Timing charts, and the “Rule of thumb at this age…” one in the Walks panel. The charts above them already say it. The age-based walking goal is still there, drawn as the goal line on the Walk trend" },
   { "date": "2026-09-01", "text": "On the dark theme the awake timers are a warm near-white now — both “Awake for” at the top and the pill in the day bar — instead of yellow text on a yellow-tinted pill. The light theme keeps its dark gold, where white would disappear into a near-white pill" },
   { "date": "2026-09-01", "text": "Toned the awake timer down: the pill in the day bar is a fainter yellow and its text a good deal darker, so “Awake for” is comfortable to read rather than technically legible. The Awake labels elsewhere darkened with it" },
   { "date": "2026-08-31", "text": "The asleep timer text is a deeper blue, so “Asleep for” in the day-bar pill and on the big card is properly readable against its own pale blue background instead of the near-invisible blue-on-blue it was — the same treatment the awake gold just got, and it applies to the Asleep labels in the Sleep & wake list too" },