rugnux: print ISa as undetermined when the error model's b is not resolved from zero

The (a, b) error-model fit already refuses to report ISa when its top intensity
bin has no leverage on b. It still printed 1/b when b had leverage but came out
at zero within its own error - the strongest bins scatter no more than
counting statistics say (the fitted b^2 is negative and clamps to 0), or there
are too few samples for the bin medians to mean anything. ISa then reports the
noise in b as an I/sigma: on one low-resolution sweep consecutive merges of the
same data gave ISa 0, 28, 46, 113 and 130.

The standard error of b^2 is taken from the 16 bins' own scatter about the
fitted line, and when b^2 is less than two standard errors above zero the
merge result carries isa_resolved = false. Only what is printed follows it:
the report's ISA key reads "undetermined", the summary line says so, the
asymptote is not printed and the mmCIF carries "?". The fitted ISa itself is
unchanged and is still what the space-group search's present-reflection cut
and the refused-point-group arbitration read, so no decision moves.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/ResultReportTest.cpp

@@ -651,6 +651,30 @@ TEST_CASE("ResultReport_FittedResolutionSuppressed", "[Diagnostics]") {
     CHECK(climbs_text.find("RESOLUTION_FIT") != std::string::npos);
 }
 
+// ISa of a b that is zero within its error is the noise in b; the report says undetermined, not a number.
+TEST_CASE("ResultReport_IsaUndetermined", "[Diagnostics]") {
+    DiffractionExperiment x(DetJF(1));
+    ProcessResult result;
+    result.consensus_cell = UnitCell{.a = 79.0f, .b = 79.0f, .c = 38.0f,
+                                     .alpha = 90.0f, .beta = 90.0f, .gamma = 90.0f};
+    result.has_merge_statistics = true;
+    result.merge_statistics.overall.d_max = 50.0f;
+    result.merge_statistics.overall.d_min = 1.50f;
+    result.merge_statistics.overall.cc_half = 0.99;
+    result.error_model_a = 1.1;
+    result.error_model_b = 1e-4;
+    result.error_model_isa = 95.0;
+    result.error_model_isa_asymptotic = 120.0;
+
+    const auto resolved = RenderResultReport("p", "in.h5", x, result);
+    CHECK(resolved.find("\nISA= 95.00\n") != std::string::npos);
+
+    result.error_model_isa_resolved = false;
+    const auto text = RenderResultReport("p", "in.h5", x, result);
+    CHECK(text.find("\nISA= undetermined\n") != std::string::npos);
+    CHECK(text.find("ISA_ASYMPTOTIC") == std::string::npos);
+}
+
 // The completeness of the range where the signal is, beside the completeness of the range that was
 // written. Users compare the single overall figure against another program's and conclude the run
 // lost data, when it is the deliberately generous cut diluting the denominator - so the report says