Address PR #356 CI review round 16 (1 P1 guide)

Rebased onto current main (17 commits clean — PR #355, #358, #359 all
merged since last rebase).

StaggeredTripleDifference corrected as panel-only + balance-enforced.
The earlier §4.10 RCS wording paired TripleDifference /
StaggeredTripleDifference together in the Explicit RCS support list,
but REGISTRY.md §StaggeredTripleDifference requires a balanced panel
and staggered_triple_diff.py:93-109 has no panel=False mode — fit()
rejects unbalanced/duplicate (unit, time) structure at
staggered_triple_diff.py:846-864.

- §4.10 Explicit RCS support: TripleDifference (two-period) only;
  StaggeredTripleDifference removed from the supported set.
- §4.10 Explicitly rejected for RCS: StaggeredTripleDifference added
  with a concrete "no panel=False mode" + "use TripleDifference for
  cross-sectional DDD" pointer.
- §3 Balanced-panel eligibility: StaggeredTripleDifference added to
  the balance-sensitive gate.

Regression tests extended:
- Balanced-panel proximity check now covers StaggeredTripleDifference.
- §4.10 section test asserts StaggeredTripleDifference appears in the
  Explicitly rejected block and NOT in the Explicit RCS support block.

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

Author: igerber

diff_diff/guides/llms-autonomous.txt

@@ -277,7 +277,8 @@ supported / out of scope; `warn` supported but with documented caveats;
 **Balanced-panel eligibility.** The following estimators require
 exactly one observation per `(unit, time)` cell with every unit
 observed in every period: `ContinuousDiD`, `EfficientDiD`,
-`SyntheticDiD`, `HeterogeneousAdoptionDiD`. Gate these on BOTH
+`SyntheticDiD`, `HeterogeneousAdoptionDiD`,
+`StaggeredTripleDifference`. Gate these on BOTH
 `PanelProfile.is_balanced == True` AND the absence of the
 `duplicate_unit_time_rows` alert (`is_balanced` is computed from the
 unique-key support and stays `True` when duplicates exist; the
@@ -510,10 +511,11 @@ Explicit RCS support in this library:
 
 - `CallawaySantAnna(panel=False)` - repeated-cross-section mode per
   REGISTRY.md §CallawaySantAnna; use this variant on RCS data.
-- `TripleDifference` / `StaggeredTripleDifference` - DDD cross-sectional
-  use cases are documented in `docs/choosing_estimator.rst`; the DDD
-  estimators do not require within-unit tracking when the third
-  comparison axis carries the identification.
+- `TripleDifference` - DDD cross-sectional use cases are documented
+  in `docs/choosing_estimator.rst`; the two-period DDD estimator does
+  not require within-unit tracking when the third comparison axis
+  carries the identification. The staggered DDD variant is panel-only
+  and listed separately below.
 
 Explicitly rejected for RCS (panel-only):
 
@@ -524,6 +526,10 @@ Explicitly rejected for RCS (panel-only):
 - `SyntheticDiD` - requires balanced panel with per-unit donor matching.
 - `ContinuousDiD` - requires balanced panel with per-unit constant
   dose.
+- `StaggeredTripleDifference` - panel-only; `fit()` has no
+  `panel=False` mode and rejects duplicate / unbalanced
+  `(unit, time)` structure. For cross-sectional DDD data use
+  `TripleDifference` instead.
 
 Treat other estimators in this guide as panel-only unless their own
 docs explicitly say otherwise. When routing, also:

tests/test_profile_panel.py

@@ -676,6 +676,7 @@ def test_guide_api_strings_resolve_against_public_api():
         "EfficientDiD",
         "SyntheticDiD",
         "HeterogeneousAdoptionDiD",
+        "StaggeredTripleDifference",
     ):
         idx = 0
         found = False
@@ -763,12 +764,32 @@ def test_guide_api_strings_resolve_against_public_api():
     rcs_section_start = text.find("§4.10 Repeated cross-sections")
     assert rcs_section_start >= 0
     rcs_section = text[rcs_section_start : rcs_section_start + 2500]
-    for panel_only in ("EfficientDiD", "HeterogeneousAdoptionDiD"):
+    for panel_only in (
+        "EfficientDiD",
+        "HeterogeneousAdoptionDiD",
+        "StaggeredTripleDifference",
+    ):
         assert panel_only in rcs_section, (
             f"§4.10 must explicitly name {panel_only!r} as panel-only "
             "so RCS data is not routed to it"
         )
 
+    # The explicit RCS-capable bullet list must NOT put
+    # StaggeredTripleDifference next to the RCS-support language.
+    # The estimator has no panel=False mode and fit() rejects
+    # unbalanced input; only TripleDifference (non-staggered) is
+    # cross-sectional-DDD-capable.
+    explicit_support_block = text.find("Explicit RCS support", rcs_section_start)
+    rejected_block = text.find("Explicitly rejected for RCS", rcs_section_start)
+    assert 0 <= explicit_support_block < rejected_block, (
+        "§4.10 must separate an Explicit RCS support list from the " "Explicitly rejected list"
+    )
+    explicit_segment = text[explicit_support_block:rejected_block]
+    assert "StaggeredTripleDifference" not in explicit_segment, (
+        "StaggeredTripleDifference must NOT appear in the Explicit RCS "
+        "support list — it is panel-only and balance-enforced."
+    )
+
 
 def test_min_pre_post_use_per_unit_observed_support():
     """On an unbalanced panel where one treated unit is missing its