build: use 1.12.13-dev

Use -dev suffix for development version; explain in README

README.org

@@ -43,6 +43,17 @@ under the name ~maildir-utils~; apparently because they don't like short names.
 All of the code is distributed under the terms of the [[https://www.gnu.org/licenses/gpl-3.0.en.html][GNU General Public License
 version 3]] (or higher).
 
+* Versions
+
+*mu* attempts to balance development speed and stability. For this, every few
+months there is a stable version (e.g. ~1.12.12~); development then continues in
+in ~master~, with ~~-dev~ (e.g. ~1.12.13-dev~): ~-dev~-suffixed versions refer to the
+current _development_ code, until it is released (and then looses its ~-dev~ suffix,
+i.e., become the ~1.12.13~ release).
+
+So if you want stability, grab the [[https://github.com/djcb/mu/releases/][release]]; if you are more
+adventurous/impatient, git ~master~ is there as well.
+
 * Installation
 
 Note: building from source is an /advanced/ subject, especially if something goes

meson.build

@@ -17,7 +17,7 @@
 ################################################################################
 # project setup
 project('mu', ['c', 'cpp'],
-        version: '1.12.12',
+        version: '1.12.13-dev',
         meson_version: '>= 0.56.0',
         license: 'GPL-3.0-or-later',
         default_options : [
@@ -28,7 +28,7 @@ project('mu', ['c', 'cpp'],
 
 # hard-code the date here (for reproducibility); we derive the dates used in
 # e.g. documentation from this.
-mu_date='2025-07-19'
+mu_date='2025-09-04'
 
 # installation paths
 prefixdir = get_option('prefix')