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/include/PStringUtils.h

@@ -104,10 +104,15 @@ class PStringUtils
      * <p>Converts the leading part of the string to a double.
      * Mirrors TString::Atof(). Returns 0.0 if no conversion is possible.
      *
+     * <p>If \a ok is non-null it is set to true when a value was parsed and
+     * to false on error (no number, or value out of range), allowing callers
+     * to distinguish a legitimate 0.0 from a failed conversion.
+     *
      * @param str string to be converted
-     * @return converted double value
+     * @param ok  optional out-parameter signalling conversion success
+     * @return converted double value (0.0 on error)
      */
-    static double ToDouble(const std::string &str);
+    static double ToDouble(const std::string &str, bool *ok = nullptr);
 
     /**
      * <p>Case-insensitive full-string equality.