Improve the overall speed and memory use of libCellML

[agarny]

Fixes #1368.

[Copilot]

Pull request overview

This PR focuses on performance/memory optimisations across libCellML (per #1368), largely by reducing copies, replacing linear searches with hash-based lookups, and removing some regex usage in hot paths.

Changes:

• Reduce copying by returning const std::string & / const std::vector<...> & from many getters and by tightening lambda captures.
• Improve algorithmic complexity in several areas (e.g., unordered_set/unordered_map for uniqueness checks/caches; pre-reserving vectors/strings; caching equivalence lookups).
• Replace some regex-based string processing with simpler/faster implementations (XML error formatting, Printer math normalisation, Generator version string handling).

Reviewed changes

Copilot reviewed 54 out of 55 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
tests/generator/generator.cpp	Adds a large-model repeated-run test block (currently commented out).
tests/bindings/python/test_generator.py	Adds a very-large-model test block but disables it via a triple-quoted string literal.
tests/bindings/javascript/generator.test.js	Adds a very-large-model test block (currently commented out) including manual delete calls.
src/xmlnode.h	Adds XmlNode::rawName() accessor for faster element-name checks.
src/xmlnode.cpp	Optimises element/attribute checks; implements rawName(); simplifies attribute access and element namespace comparison.
src/xmldoc.cpp	Replaces regex-based newline normalisation with std::replace.
src/xmlattribute.cpp	Speeds up namespace comparisons by using libxml namespace pointers directly and handling null namespaces explicitly.
src/variable.cpp	Minor capture/perf tweaks; changes some getters to return const std::string &.
src/validator.cpp	Reworks many uniqueness checks to hash sets; optimises MathML element support checks; refactors unit-equivalence logic.
src/utilities.h	Changes analyserVariables(AnalyserVariablePtr) to return a const reference.
src/utilities.cpp	Adds reserve() in helpers; switches some operations to lower-overhead implementations; updates analyser-variable collection helpers.
src/units.cpp	Minor lambda capture improvements; avoids extra copies in unitAttributes and setters.
src/reset.cpp	Changes string getters to return const std::string &.
src/printer.cpp	Removes regex use; optimises connection serialisation and whitespace cleanup; reduces repeated string copies; dedupes imports more efficiently.
src/parser.cpp	Avoids unnecessary copies when iterating namespace maps.
src/namedentity.cpp	Changes name() to return const std::string &.
src/model.cpp	Minor lambda capture improvements; uses a set to avoid duplicate import requirement URLs.
src/issue.cpp	Changes description() to return const std::string &.
src/internaltypes.h	Switches several internal container typedefs to unordered variants (and changes IdList type).
src/importsource.cpp	Changes url() to return const std::string &.
src/importer.cpp	Uses find/emplace patterns for maps/sets; reduces repeated lookups/copies; optimises name-collision checks.
src/importedentity.cpp	Changes importReference() to return const std::string &.
src/generatorvariabletracker.cpp	Reduces temporary vectors and reserves before inserting.
src/generatorprofile.cpp	Changes many profile string getters to return const std::string &.
src/generator.cpp	Reduces string churn via addCode, reserve(), fewer temporaries; replaces regex semver tweak with manual scan; uses hash sets for equation bookkeeping.
src/generator_p.h	Updates generator internals to use unordered_set parameters and replaces newline helper with addCode.
src/entity.cpp	Changes id() to return const std::string &.
src/componententity.cpp	Minor lambda capture improvements; changes encapsulationId() to return const std::string &.
src/component.cpp	Minor lambda capture improvements; changes math() to return const std::string &.
src/commonutils.cpp	Simplifies owningModel() traversal to walk parents directly.
src/CMakeLists.txt	Adds CODE_COVERAGE_ENABLED compile definition when coverage is enabled.
src/api/libcellml/variable.h	Updates public API getters to return const std::string & for initial value and interface type.
src/api/libcellml/reset.h	Updates public API reset getters to return const std::string &.
src/api/libcellml/namedentity.h	Updates public API name() to return const std::string &.
src/api/libcellml/issue.h	Updates public API description() to return const std::string &.
src/api/libcellml/importsource.h	Updates public API url() to return const std::string &.
src/api/libcellml/importedentity.h	Updates public API importReference() to return const std::string &.
src/api/libcellml/entity.h	Updates public API id() to return const std::string &.
src/api/libcellml/componententity.h	Updates public API encapsulationId() to return const std::string &.
src/api/libcellml/component.h	Updates public API math() to return const std::string &.
src/api/libcellml/analysermodel.h	Updates public API analyser-model vector getters to return const std::vector<...> &.
src/api/libcellml/analyserexternalvariable.h	Updates public API dependencies() to return const std::vector<...> &.
src/api/libcellml/analyserequationast.h	Updates public API value() to return const std::string &.
src/api/libcellml/analyserequation.h	Updates public API analyser-equation vector getters to return const std::vector<...> &.
src/annotator.cpp	Avoids repeated owningComponent() calls; fixes switch to use already-computed type.
src/analyservariable.cpp	Reserves result size when expanding weak equation references.
src/analysermodel.cpp	Returns vectors by const ref with static empties on invalid; adds caches for analyser-variable lookup and variable equivalence results.
src/analysermodel_p.h	Replaces map-based cache with unordered_map keyed by pointer-pairs + hash; adds analyser-variable cache.
src/analyserexternalvariable.cpp	Tightens lambda captures; returns dependencies by const reference.
src/analyserequationast.cpp	Returns AST value by const reference.
src/analyserequation.cpp	Reserves when expanding weak refs; returns several vectors by const reference.
src/analyser.cpp	Adds internal pointer caches/sets; swaps MathML element checks to rawName()+strcmp path for speed; reduces duplicate scanning in multiple places.
src/analyser_p.h	Adds set/cache members and switches several internal maps to unordered variants.

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