resolver: drop the unused timeout multiplier and the comment claiming it (closes #296)

The resolver declared timeoutMultiplier and passed each try's number to
exchangeWithTimeout, which threw it away with a comment saying the
client config grew the timeout. Nothing did: every try waits the same
2-second queryTimeoutDuration. The constant goes, and exchangeWithTimeout
is folded into its one caller, tryExchange, whose comment now says the
timeout is fixed. Behaviour is unchanged.

Model: opus-5-5

TODO.md

@@ -19,6 +19,9 @@ milestone: https://git.eeqj.de/sneak/dnswatcher/milestone/7
 
 # Completed Steps
 
+- 2026-10-15: the resolver no longer says its query timeout grows on each try;
+  every try waits the same 2 seconds, and the unused `timeoutMultiplier` is gone
+  (closes #296).
 - 2026-10-15: `TESTING.md` Constraints give the resolver's 2-second query
   timeout and second try, say a zone's servers are asked one at a time in a
   random order, give the 60-second cap and 20-second target for `make test`, and