ci(pyamber): measure branch coverage

.github/workflows/build.yml

@@ -1122,6 +1122,12 @@ jobs:
         # --junit-xml emits a JUnit-XML report alongside the coverage XML
         # so the Test Analytics upload below can feed Codecov's failing-
         # test PR comments and flaky-test detection on main.
+        #
+        # Branch coverage is ON, but via `[tool.coverage.run] branch = true`
+        # in amber/pyproject.toml rather than a `--cov-branch` flag here, so
+        # that a local `pytest --cov` reports the same metric as CI. Don't
+        # add the flag back: it would be redundant, and two switches for one
+        # boolean invites someone to remove one and think it is off.
         env:
           # Pytest runs with -s, so loguru's default stderr sink prints
           # straight into the CI log; pin it to WARNING (loguru has no WARN)

amber/pyproject.toml

@@ -42,4 +42,13 @@ testpaths = ["src/test/python"]
 addopts = "--import-mode=importlib"
 markers = [
     "integration: end-to-end test routed to the amber-integration CI job",
-]
+]
+
+# Branch coverage. Without this, coverage.py records only whether each
+# statement ran, so a half-taken `if` reads as fully covered and Codecov
+# has no partial-line data for any pyamber file. Setting it here rather
+# than passing `--cov-branch` in CI keeps a local `pytest --cov` run on
+# the same metric as the pipeline — a local-vs-CI split is exactly how a
+# line comes to look green on a laptop and partial on Codecov.
+[tool.coverage.run]
+branch = true