cleanup: assert on flat _zscore, doc the nll floor, trim history comments

_zscore now asserts nonzero variance (a flat profile drew a degenerate all-zero
map that looked valid) instead of dividing by std+1e-9. _soft_nll docstring now
states the 1e-12 floor it actually applies (was 'Unbounded'). Trim archaeology
comments (guided/wvs/read_api) to the invariant, dropping the dated model names.

Co-Authored-By: Claudypoo <288921227+claudypoo@users.noreply.github.com>

scripts/wvs_map.py

@@ -169,8 +169,8 @@ def main() -> None:
                                               for k, v in cache.items()}
 
     def save_cache() -> None:
-        """Persist after EACH model so a killed run (session teardown) keeps finished models -- the
-        write-once-at-end version silently discarded every sampled model when interrupted."""
+        """Persist after EACH model so a killed run (session teardown) keeps every finished model
+        (kill-safe incremental write, not write-once-at-end)."""
         allc = json.loads(cpath.read_text()) if cpath.exists() else {}
         allc[sig] = {k: {s: p.tolist() for s, p in v.items()} for k, v in vecs.items()}
         cpath.write_text(json.dumps(allc))