fix(s3-followup): enforce V5 rule 4 + mark H3/H4/H5/V5/V6 done

serialization.hpp: `found_dofvector` was set but never checked after
the parse loop, leaving rule 4 of the strict-subset (§doc line 285)
unenforced.  Add the missing post-loop throw so a ConformalResult XML
that omits the <DOFVector> element is rejected with a clear error
instead of silently returning an empty x.  Update the docstring to
remove the misleading "silently returns" note.

finding-orchestration.md: mark H3/H4/H5/V5/V6 ✅ and record the S3
session as complete.  Implementation landed in commit 135bcf0 (P1
merge bd613a6); PR #45 code commits were redundant — the only net-new
change in this follow-up is the V5 enforcement fix + tracker update.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

code/include/serialization.hpp

@@ -283,8 +283,8 @@ inline void save_result_xml(
 ///  3. A line containing `<DOFVector` must carry the `>` character (tag
 ///     open) on the same line.
 ///  4. The `<DOFVector` element must be present and must produce a
-///     non-empty doubles list (a missing DOFVector silently returns an
-///     empty x, which is incorrect for any mesh with at least one DOF).
+///     non-empty doubles list (a missing DOFVector element causes a
+///     `std::runtime_error` — enforced after the parse loop).
 inline std::vector<double> load_result_xml(
     const std::string& path,
     NewtonResult*       res      = nullptr,
@@ -386,6 +386,13 @@ inline std::vector<double> load_result_xml(
                 " Only the format written by save_result_xml is supported.");
     }
 
+    // V5 rule 4: <DOFVector> must be present in every well-formed ConformalResult.
+    if (found_root && !found_dofvector)
+        throw std::runtime_error(
+            "conformallab: XML strict-subset violation in " + path
+            + ": <DOFVector> element not found. Only the format written by"
+            " save_result_xml is supported.");
+
     return x;
 }