test(audit): behavioral SSDL test locks down effective_codepaths math

Adds a small synthetic fixture (tests/fixtures/synthetic_ssdl/) with 5
consumer functions, each containing 3 explicit if-statements. The fixture
is self-contained and does not depend on the live src/ tree.

The new test tests/test_code_path_audit_ssdl_behavioral.py has 2 tests:
- test_effective_codepaths_synthetic: builds an AggregateProfile with 5
  consumers pointing at the fixture's 5 functions, calls
  compute_effective_codepaths, asserts the result is 40 (= 5 consumers x
  2^3 branches per function).
- test_effective_codepaths_candidate_returns_zero: asserts that an
  AggregateProfile with is_candidate=True returns 0 (the SSDL early-exit
  guard for candidate aggregates).

This locks down the SSDL effective-codepaths math so future refactors of
compute_effective_codepaths() or count_branches_in_function() cannot
silently change the formula without a failing test.

Verification:
- uv run pytest tests/test_code_path_audit_ssdl_behavioral.py -v -> 2 passed

tests/fixtures/synthetic_ssdl/sample_module.py

@@ -0,0 +1,48 @@
+def func_one(x: int) -> int:
+ if x > 0:
+  x += 1
+ if x > 1:
+  x += 1
+ if x > 2:
+  x += 1
+ return x
+
+
+def func_two(x: int) -> int:
+ if x > 0:
+  x += 1
+ if x > 1:
+  x += 1
+ if x > 2:
+  x += 1
+ return x
+
+
+def func_three(x: int) -> int:
+ if x > 0:
+  x += 1
+ if x > 1:
+  x += 1
+ if x > 2:
+  x += 1
+ return x
+
+
+def func_four(x: int) -> int:
+ if x > 0:
+  x += 1
+ if x > 1:
+  x += 1
+ if x > 2:
+  x += 1
+ return x
+
+
+def func_five(x: int) -> int:
+ if x > 0:
+  x += 1
+ if x > 1:
+  x += 1
+ if x > 2:
+  x += 1
+ return x