Cover stratified-no-PSU branch in scaffolding equivalence tests

Addresses the P2 test-coverage gap from the CI review on PR #338:
_precompute_psu_scaffolding() has a dedicated strata-with-no-PSU
branch (survey.py:1458-1466) where each observation is its own PSU
within its stratum, but TestAggregateSurveyScaffolding did not
exercise it despite the docstring claiming every supported design
mode was covered.

Adds two parametrized cases to test_fast_path_equals_legacy:
- stratified_no_psu        — SurveyDesign(weights=, strata=)
- stratified_no_psu_fpc    — same plus stratum-level FPC lookup
(the fpc-on-this-branch path goes through the same per-stratum
first-obs FPC read as stratified+PSU, so both variants matter).

Both pass assert_allclose(atol=1e-14, rtol=1e-14) equivalence with
the legacy path across all cells.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

tests/test_prep.py

@@ -3483,6 +3483,24 @@ def _build_microdata(self, mode, seed=42):
             sd = SurveyDesign(weights="wt", strata="stratum", psu="psu")
             return df, sd
 
+        if mode == "stratified_no_psu":
+            # strata present, psu absent — each observation is its own
+            # PSU within its stratum.  This is a distinct scaffolding
+            # branch (survey.py:_precompute_psu_scaffolding, else clause
+            # of the `if psu is not None` block).
+            df = df_base.copy()
+            df["stratum"] = rng.integers(0, 4, n)
+            sd = SurveyDesign(weights="wt", strata="stratum")
+            return df, sd
+
+        if mode == "stratified_no_psu_fpc":
+            # Same branch as above plus stratum-level FPC lookup.
+            df = df_base.copy()
+            df["stratum"] = rng.integers(0, 4, n)
+            df["fpc"] = 1000.0  # well above per-stratum obs count
+            sd = SurveyDesign(weights="wt", strata="stratum", fpc="fpc")
+            return df, sd
+
         if mode == "psu_only":
             df = df_base.copy()
             df["psu"] = rng.integers(0, 12, n)
@@ -3532,6 +3550,8 @@ def _assert_panels_equivalent(p_fast, p_legacy, outcome="y"):
         [
             "stratified_fpc",
             "stratified_no_fpc",
+            "stratified_no_psu",
+            "stratified_no_psu_fpc",
             "psu_only",
             "weights_only",
             "lonely_remove",