fix(build): keep editable version fallback out of wheels

The existing release stamper matches every line containing `__version__ =`.
A comparison with the version variable on the left is also matched, causing
it to replace the placeholder in the new editable-import guard. That would
incorrectly enable the source-tree `VERSION` lookup in a built wheel, which
does not ship that file.

Put the literal on the left of the guard so stamping changes only the public
assignment. Extend the existing isolated installation test to verify public
version parity with the running source checkout.

The isolated installation and version checks pass with both CLI and Gix;
Ruff lint/format and `git diff --check` pass. The full CLI suite also passes:
1,827 passed,162 skipped,1 xfailed and59 passed subtests.

Author: codex

git/__init__.py

@@ -81,7 +81,7 @@
 ]
 
 __version__ = "git"
-if __version__ == "git":
+if "git" == __version__:
     # Wheels and source distributions stamp this value; editable imports do not.
     from pathlib import Path as _Path
 

test/test_installation.py

@@ -7,7 +7,7 @@
 import subprocess
 import sys
 
-from git import _backend
+from git import __version__, _backend
 from test.lib import TestCase, requires_symlinks
 from test.lib.fixtures import ROOT, writable_fixture
 
@@ -44,6 +44,10 @@ def _test_installation(self, environment):
         self._check_result(result, "Backend selection failed")
         self.assertEqual(result.stdout.strip(), _backend.name)
 
+        result = run([python, "-c", "import git; print(git.__version__)"])
+        self._check_result(result, "Installed version import failed")
+        self.assertEqual(result.stdout.strip(), __version__)
+
         result = run([python, "-c", "import gitdb; import smmap"])
         self._check_result(result, "Dependencies not installed")