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/style.css

@@ -390,7 +390,6 @@ button.danger { background: var(--danger); }
   font-size: 0.8rem;
   font-weight: normal;
 }
-.timing-hint { margin: 10px 4px 0; line-height: 1.4; }
 
 /* ---------- timing range charts ---------- */
 .timing-charts {