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/index.html

@@ -215,7 +215,6 @@
         <!-- The axis is stretched (see drawTimingChart), so say what the middle
              of a bar means rather than leave it to be inferred. -->
         <p class="muted-note">The middle of every bar is that type's typical gap: left of it is sooner than usual, right of it is longer, and the faded stretch runs out to the longest gap in the window.</p>
-        <p class="muted-note timing-hint" id="timing-hint"></p>
       </section>
 
       <!-- Sleep and wake windows are the same boundaries read two ways — a wake
@@ -290,9 +289,6 @@
         <h2>Walks <span class="muted-note" id="walk-total"></span></h2>
         <ul id="walk-list" class="wake-list"></ul>
         <p id="walk-empty" class="empty">No walks yet for this day. Use 🦮 Walk start / 🏁 Walk end to time one.</p>
-        <!-- Age-based guidance ("the five-minute rule"), only when a birthday
-             is set and the puppy is still growing. -->
-        <p id="walk-goal" class="muted-note" hidden></p>
         <div class="chart walk-chart" id="walk-chart-wrap" hidden>
           <div class="chart-title">Minutes per day <span data-chart-days-label>(last 7 days)</span></div>
           <svg id="chart-walk" class="chart-svg" viewBox="0 0 320 160" role="img" aria-label="Minutes walked per day"></svg>