1.7.0: shipping rule "Only" reserves a method, "Show" no longer does

Since 1.6.0 "Show" both whitelisted a method for its group and hid it from
everybody else. A group shown ordinary methods next to a free one therefore
took the ordinary ones away from every stranger, who was left with only the
free method. "Only" now carries the reservation; "Show" is just the group's
whitelist. Existing show rules are converted to only once on update so the
checkout does not change until the shop chooses.

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

docs/RELEASE.md

@@ -1,6 +1,6 @@
 # Releasing
 
-One release a day at most, cut by hand. Version 1.6.3 is current.
+One release a day at most, cut by hand. Version 1.7.0 is current.
 
 ## The sequence
 
@@ -233,7 +233,7 @@ derives everything from the header, so splitting them was a one-line change —
 and it means the source repository's visibility is a separate decision from the
 feed's.
 
-⚠️ The source repository is **still public** as of 1.6.3. Making it private is
+⚠️ The source repository is **still public** as of 1.7.0. Making it private is
 safe only once no installation is still pointing its `Update URI` at the old
 address; an install on a pre-1.5.0 build reads its feed from
 `kw/nolimit-discounts`, and making that private would silently stop its