PStringUtils::ToInt: signal conversion errors via from_chars

ToInt() previously wrapped strtol() with a nullptr endptr, so a failed
conversion was indistinguishable from a legitimate 0 (matching the old
TString::Atoi() behaviour). Switch the implementation to std::from_chars
and add an optional `bool *ok` out-parameter that reports success: it is
set to false on a non-numeric string or an out-of-range value, true
otherwise. Leading whitespace is skipped and trailing characters are
ignored, preserving the Atoi-like prefix semantics. The parameter
defaults to nullptr, so existing call sites keep compiling unchanged.

Convert the parse-validation call sites in PMsrHandler to the single
-parse ToInt(token, &ok) form, replacing the IsInt() guard + separate
ToInt() (which parsed every token twice). All downstream >0 / >=0 /
range / enum checks are preserved.

Left untouched the call sites where IsInt() acts as a structural
discriminator rather than a numeric validator (write path, xy-data
index-vs-label, fParamInUse usage scans) and the IsFloat-guarded ToInt
offsets, where switching to ToInt(&ok) would change parsing semantics.

All 85 integration tests pass.

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

src/include/PStringUtils.h

@@ -89,10 +89,16 @@ class PStringUtils
      * <p>Converts the leading part of the string to an int (base 10).
      * Mirrors TString::Atoi(). Returns 0 if no conversion is possible.
      *
+     * <p>If \a ok is non-null it is set to true when a valid integer was
+     * parsed and to false on error (no digits, or value out of int range).
+     * This allows callers to distinguish a legitimate 0 from a failed
+     * conversion, which the bare return value cannot express.
+     *
      * @param str string to be converted
-     * @return converted integer value
+     * @param ok  optional out-parameter signalling conversion success
+     * @return converted integer value (0 on error)
      */
-    static int ToInt(const std::string &str);
+    static int ToInt(const std::string &str, bool *ok = nullptr);
 
     /**
      * <p>Converts the leading part of the string to a double.