fix(test): LAST_SHIPPED stays 17 until 7.16 actually ships

My own U5 update asserted STORAGE_VERSION_LAST_SHIPPED == 20, which is wrong in
the specific way storage.h warns about two lines above the constant:

    "Bump this baseline when a release ships, in the release commit, never to
     make a build compile: lowering it is the exact edit that turns every
     upgrade in the field into a silent wipe."

LAST_SHIPPED is the high-water mark of what is IN THE FIELD, not of what sits
in the tree. 7.15 shipped V17; 7.16 has shipped nothing. Asserting 20 would
have forced the next person to raise the baseline to make the test pass -- the
edit the gate exists to prevent, arrived at by way of the gate itself.

The compile-time assert only requires STORAGE_VERSION >= LAST_SHIPPED, and
20 >= 17 holds, so the raise was never needed. It moves in the release commit
that tags 7.16.

Author: BitHighlander

tests/test_storage_version_gate.py

@@ -442,7 +442,18 @@ def test_active_flash_format_is_v20(self):
             "is a deliberate release act (docs/StorageVersionGate.md): confirm "
             "the reader chain, the anti-rollback story, and the release notes, "
             "then update this test." % self.version)
-        self.assertEqual(20, self.last_shipped)
+        # LAST_SHIPPED stays at 17 until 7.16 actually SHIPS in a signed
+        # release. It is the high-water mark of what is IN THE FIELD, not of
+        # what is in the tree -- and storage.h says two lines above the
+        # constant that raising it to make a build compile "is the exact edit
+        # that turns every upgrade in the field into a silent wipe". The
+        # compile-time assert only needs STORAGE_VERSION >= LAST_SHIPPED, and
+        # 20 >= 17 holds, so nothing requires the raise.
+        self.assertEqual(
+            17, self.last_shipped,
+            "STORAGE_VERSION_LAST_SHIPPED is %d. It tracks the last SIGNED "
+            "release (7.15 = V17) and moves in the release commit that tags "
+            "7.16, not when a format lands in the tree." % self.last_shipped)
 
     def test_burned_versions_have_no_reader(self):
         """18 and 19 must never be parsed by 7.16.