fix(coverage): resolve fail-under from pyproject in threshold-enforcement step (WS5)

The coverage_report noxfile session now delegates COVERAGE_THRESHOLD to the
pyproject single source (`COVERAGE_THRESHOLD = _read_coverage_fail_under()`),
so the AST value is a Call, not a Constant. The
coverage_threshold_enforcement.feature step "I parse the COVERAGE_THRESHOLD
constant from noxfile.py" raised ValueError (errored scenario) because it only
handled a literal constant — this was the missed third WS5 step file (the
config + consolidated steps were already reconciled).

Add the same pyproject `[tool.coverage.report].fail_under` fallback used by
coverage_threshold_config_steps.py, and fix the phantom-97 in the feature
description prose. Verified: the four coverage feature files now pass
(136 scenarios, 0 errored); previously unit_tests errored on
coverage_threshold_enforcement.feature:7.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

features/coverage_threshold_enforcement.feature

@@ -1,5 +1,5 @@
 Feature: Coverage threshold enforcement
-  The project enforces a minimum 97% code coverage threshold.
+  The project enforces a minimum 96.5% code coverage threshold.
   This feature validates that the coverage configuration and nox session
   are properly set up to enforce this requirement.
 

features/steps/coverage_threshold_enforcement_steps.py

@@ -41,6 +41,18 @@ def step_ci_workflow_exists(context: Any) -> None:
     context.ci_file_content = ci_file.read_text(encoding="utf-8")
 
 
+def _pyproject_fail_under() -> float | None:
+    """The canonical coverage floor: pyproject ``[tool.coverage.report]``
+    ``fail_under`` (the single source the noxfile delegates to via
+    ``_read_coverage_fail_under()``). None if absent/unreadable."""
+    pyproject = PROJECT_ROOT / "pyproject.toml"
+    try:
+        data = tomllib.loads(pyproject.read_text(encoding="utf-8"))
+        return float(data["tool"]["coverage"]["report"]["fail_under"])
+    except (OSError, KeyError, TypeError, ValueError, tomllib.TOMLDecodeError):
+        return None
+
+
 @when("I parse the COVERAGE_THRESHOLD constant from noxfile.py")
 def step_parse_threshold(context: Any) -> None:
     content = context.noxfile_content
@@ -55,8 +67,16 @@ def step_parse_threshold(context: Any) -> None:
                     and isinstance(node.value, ast.Constant)
                 ):
                     threshold = node.value.value
+    # WS5: the noxfile now delegates the constant to pyproject's single source
+    # (``COVERAGE_THRESHOLD = _read_coverage_fail_under()``), so the AST value is
+    # a Call, not a Constant. Resolve the effective floor from pyproject.
     if threshold is None:
-        raise ValueError("COVERAGE_THRESHOLD constant not found in noxfile.py")
+        threshold = _pyproject_fail_under()
+    if threshold is None:
+        raise ValueError(
+            "COVERAGE_THRESHOLD not found in noxfile.py and pyproject "
+            "[tool.coverage.report].fail_under is unreadable"
+        )
     context.coverage_threshold = threshold