fix: the defective xprv wallet notice says the user needs their own copy of the key (closes #255)

The notice told the user that these addresses are reachable by software
that repeats the same derivation, while the same wallet blocks Export
Private Key, so there was no way to take that route from here. The
second paragraph now names private key export among the disabled
controls and says the addresses can be reached only with the extended
private key the user imported, if they kept a copy of it. No behaviour
changes. A test checks the new wording.

Model: opus-5-5

TODO.md

@@ -2,9 +2,10 @@
 
 - `git pull` `next` and cut a branch from it — one branch per issue, named
   `issue-<N>-<slug>`. Never branch from `main`.
-- Do the work as one commit whose title ends with ` (closes #N)`.
-- Add no entry to Completed Steps. What landed is recorded on the tracker and in
-  `git log`, where each commit title names the issue it closes.
+- Do the work as one commit whose title ends with ` (closes #N)`, with the
+  `TODO.md` update in that same commit.
+- Move Next Step to the top of Completed Steps; move the top item of Future
+  Steps into Next Step.
 - Run `make fmt`, then `make check`. A feature branch may be red; `next` and
   `main` may not.
 - Rebase onto current `next` immediately before pushing — other branches land on