PStringUtils::ToDouble: signal conversion errors like ToInt

ToDouble() had the same shortcoming as ToInt(): wrapping strtod() with a
nullptr endptr made a failed conversion indistinguishable from a
legitimate 0.0. Add an optional `bool *ok` out-parameter that reports
success. strtod() (with endptr + errno) is kept instead of
std::from_chars so the accepted input set stays identical to
TString::Atof() (leading whitespace skipped, leading '+' honoured,
trailing characters ignored); ok is set false on a non-numeric string or
an ERANGE overflow. The parameter defaults to nullptr, so existing call
sites keep compiling unchanged.

Convert the IsFloat-guarded ToDouble call sites in PMsrHandler to the
single-parse ToDouble(token, &ok) form (replacing the IsFloat() guard +
separate ToDouble() that parsed every token twice). All downstream
>=0 / <=0 / range checks are preserved, and push_back sites only append
on success so no spurious 0.0 is stored on error. Number-vs-keyword
discriminators (pos.error/boundary "none", rrf_phase/fourier-phase parX)
are restructured so the keyword branch is taken when ok is false.

As a side effect this fixes a latent gap in the GLOBAL rrf_freq handler,
where a non-numeric frequency previously slipped through with a stale
value instead of raising an error.

The IsFloat-guarded ToInt fit-range offsets (fgb/lgb) are intentionally
left untouched, as there the guard type differs from the conversion.

All 85 integration tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/classes/PMsrHandler.cpp

@@ -4807,6 +4807,8 @@ Bool_t PMsrHandler::HandlePlotEntry(PMsrLines &lines)
                 param.fRRFPhase = fParam[no-1].fValue;
               }
             }
+          } else {
+            error = true;
           }
         }
       } else if (iter1->fLine.Contains("rrf_packing", TString::kIgnoreCase)) {