refactor: use new string type part 1

[nickvunen]

What's done

Foundation (compile-time safety + helpers)

• Bumped C++ standard to C++20
• Added __attribute__((format(printf,...))) to CDBDebug, CDBError, CDBWarning, CT::printf, etc. — compiler now rejects %s against a std::string at build time
• Added 25 free helpers in namespace CT:: operating on std::string (split, trim, replace, printf, startsWith, endsWith, indexOf, toFloat, isFloat, encodeXml, basename, ...)
• Added CT::fromCStr(const char*) for null-safe const char* → std::string conversion (replaces CT::string's null-pointer-tolerant constructor)
• Added CT::string ↔ std::string interop (constructor + explicit cast)
• 23 new unit tests, 34 total passing

Migrated to std::string

Fields:

• GeoParameters::crs
• ProjectionMapKey::{sourceCRS, destCRS}
• PointDV::id
• CRectangleText::{text, fontFile}
• CServerParams::{responceCrs, mapTitle, mapSubTitle, BGColor, JSONP, queryStrURLParam, datasetLocation}
• CRequest::{Version, Exceptions}
• COpenDAPHandler::httpHeaderContentType
• CNetCDFDataWriter::tempFileName
• CXMLGen::serviceInfo
• CReporter::_filename
  Function signatures:
• isLonLatProjection, isMercatorProjection
• findReaderByFileName
• checkIfFileMatchesLayer
• findLayerConfigForRequestedLayer
• CSLD::{parameterIsSld, processSLDUrl}
• CReporter::filename
  Null-safety: .copy(rawCharPtr) → = CT::fromCStr(rawCharPtr) across CCDFDataModel/

Bug fixes uncovered by the format attribute

• CXMLSerializerInterface.cpp:102 — CT::string passed via %s varargs
• CCDFNetCDFIO.cpp:552 — CT::string * passed via %s varargs
• COpenDAPHandler.cpp:423 — getenv() null-pointer crash post-migration

Docs

• NEWS.md updated
• doc/developing/string-migration.md — patterns + risks for future PRs

Stats

• 56 files, +765 / −156 lines
• Unit tests: 34/34 pass
• Integration tests: 258 pass (was 216 pre-session; foundation fixed 42 previously-failing tests)
• Same 4 known cross-platform failures remain (proj/cairo anti-aliasing on macOS arm64)

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What's left

• CCDFDataModel domain model — CDF::Variable::name, CDF::Attribute::name, CDF::Dimension::name still CT::string. Touched by every reader/converter; needs a dedicated PR.
• CDataSource members — nativeProj4, nativeEPSG, headerFilename, meaning, etc. High ripple, needs dedicated PR.
• Auto-generated XSD layer — adagucserverEC/CServerConfig_CPPXSD.h (hundreds of CT::string attribute fields) and the field rename CXMLObjectInterface::value → elementValue. PR Improve cfg parsing #700 has a proposed mechanical rewrite for this.
• Other large carriers — LayerMetadataType.h, CKeyValuePair, COGCDims, CXMLParser::XMLElement, CTime, CDBStore::Record, CXMLGen::* (CT::string *XMLDoc builder pattern).
• Remaining hclasses/ — CXMLParser, CDirReader, CHTTPTools, CReportMessage.
• Final cleanup (Phase 6) — once all callers are migrated:
  • Delete CT::string, CT::stringref, CT::StackList
  • Remove operator const char *() (currently catches printf misuse via the format attribute, but should ultimately be replaced by all callers using .c_str() explicitly)