lib.version.Version.next(): Step the last part

Range expansion needs a bound that steps the last existing part of a
version, add that.

next() increments the last part, whatever it is: next of '1' is '2', of
'1.0' is '1.1', of '1.2.3' is '1.2.4', of '1.2.3-45' is '1.2.3-46'.

Tests written by AI.

Signed-off-by: Jan Lindemann <jan@janware.com>
Assisted-by: unsloth/Qwen3.8-27B-GGUF:Q4_K_M with pi.dev v0.85.1

src/python/jw/pkg/lib/version/Version.py

@@ -220,3 +220,28 @@ class Version:  # export
             f'{self.major}.{self.minor + 1}.0',
             self.__lookup_version,
         )
+
+    @property
+    def next(self) -> Version:
+        """The next version in the series: the last part
+        incremented. next of '1' is '2', of '1.0' is '1.1', of
+        '1.2.3' is '1.2.4', of '1.2.3-45' is '1.2.3-46'. The
+        separators of the spec are kept. Like in
+        next_binary_compatible(), only the last part's leading
+        digits count: a suffixed part such as '4blah' steps to
+        '5'. A part without leading digits raises Version.Error.
+        """
+        untemplated = self.__id(untemplate = True, throw = True)
+        parts = re.split('([.-])', untemplated)
+        m = re.match('[0-9]+', parts[-1])
+        if not m:
+            raise Version.Error(
+                f'Cannot step version "{self.__spec}": the last '
+                f'part "{parts[-1]}" has no leading digits'
+            )
+        parts[-1] = str(int(m.group()) + 1)
+        return Version(
+            self.__name,
+            ''.join(parts),
+            self.__lookup_version,
+        )

test/unit/python/jw/pkg/lib/version/test.py

@@ -193,6 +193,29 @@ for spec in ['1.2.3-rc1', '1.2.3']:
     except Version.Error:
         pass
 
+# next steps the last existing part, whatever it is
+for spec, expected in [
+    ('1', '2'),
+    ('1.0', '1.1'),
+    ('1.2.3', '1.2.4'),
+    ('1.2.3-45', '1.2.3-46'),
+    ('1.2.3-4blah', '1.2.3-5'),
+    ('1.2.3.4', '1.2.3.5'),
+    ('1.2.3rc1', '1.2.4'),
+    ('VERSION', '1.2.4'),
+]:
+    v = Version('foo', spec, app.get_version)
+    assert str(v.next) == expected, spec
+
+# A last part without leading digits cannot be stepped
+for spec in ['1.0-rc1', '1.2.alpha']:
+    v = Version('foo', spec)
+    try:
+        v.next
+        assert False, f'Should have raised for {spec!r}'
+    except Version.Error:
+        pass
+
 # constructor
 d = Dependency('foo-devel = 1.0')
 assert d.base_name == 'foo'