fix: review pass - Discover's late replies dropped and its way back usable, the Gallery's newest first and faster, the network clock's toast

- Discover: a reply after its try timed out, was stopped or was overtaken
  went on to the normal handling and became the contact's route (and sent
  a path back); the last tags sent are kept and a late reply is dropped.
  Leaving Route on the L2 stops a discovery as it stops a trace.
- Discover: a route whose reply came back another way offers that way too
  (L2 There / Back, L1 a Back row), turned round into a route there, so a
  trace checks it; the Discover button no longer sits on its group.
- Screenshots: numbered one past the highest (one pass over the folder,
  not a stat per number), so one taken after a delete is still the newest.
- Gallery: the newest 300 kept whatever order the folder lists them in;
  BMP rows read in the file's order and several at a time; the thumbnail
  buffer sized to the page; the folder read once per visit; a failed
  open frees or restores the picture.
- NTP: the slot the router's server goes in emptied before joining, so a
  cached address isn't taken for it; "Clock set from the network" judged
  against the clock a tick before (SNTP has set the system time already).
- Sim: sim_scroll up stops at the top.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/solo/contacts.md

@@ -94,7 +94,10 @@ on the map. On the **Discover** row, **Left / Right** sets 1, 3 or 5 tries
 and **Enter** starts it (or stops it). Each try waits for its reply or its
 timeout, then 20 seconds more, so the floods don't come all at once. Each
 route found is listed under it with how many times it came; **Enter** on one
-puts it in the list, ready to **Trace** and save. A repeater you don't have
+puts it in the list, ready to **Trace** and save. When the reply came back
+another way, a **Back** row follows it: that way turned round, as a route to
+the node, so a trace checks it too (a trace goes there and back on the same
+repeaters). A repeater you don't have
 as a contact shows as its hash (`?8D`). Nothing is saved until you save it.
 
 Only a node that answers requests can be asked: a companion whose
@@ -119,7 +122,8 @@ straight to the node. The route a node sends back usually replaces it again;
 > **Discover** is under the route, in **Find the route**, with the number of
 > tries beside it and where it is (the try waiting for its reply, or the
 > seconds to the next one). Tap a route found to use it; the way back is under
-> it.
+> it. When the way back is another one, **There** and **Back** put either in
+> the list to trace.
 
 ## Your own contact