style: unify identifier naming per updated conventions

Automated with clang-tidy readability-identifier-naming (config added to
.clang-tidy) plus scripted passes, per the updated rules now documented
in CONTRIBUTING.md:

- types (class/struct/enum/alias/template params): PascalCase
- functions, variables, members: snake_case (incl. rational -> Rational)
- private/protected members: trailing underscore; static member
  variables likewise (instance_, available_themes_)
- constants and enum values: snake_case (kLinear -> k_linear,
  F32P -> f32p); ALL_CAPS reserved for macros
- macros: OAK_ prefix (OLIVE_ADD_TEST/OLIVE_ASSERT/OLIVE_CONFIG ->
  OAK_ADD_TEST/OAK_ASSERT/OAK_CONFIG, GL_PREAMBLE -> OAK_GL_PREAMBLE,
  include guards -> OAK_*)
- file names: all lowercase (Current/Plugin/OliveHost/OliveClip/
  OlivePluginInstance -> current/plugin/olivehost/oliveclip/
  oliveplugininstance)
- getters share the member name sans underscore, setters set_foo()
- Qt and third-party (OpenFX) virtual overrides and framework callbacks
  keep their original names (exempt in .clang-tidy)

Manual follow-ups required where automation could not reach:
- string-based QMetaObject/SIGNAL/SLOT references updated to renamed
  methods (AddTask, CreatedFile, DeleteSpecificFile, moveSelectionUp, ...)
- macro bodies referencing renamed methods (OLIVE_CONFIG,
  NODE_DEFAULT_DESTRUCTOR, MANAGEDDISPLAYWIDGET_*)
- self-shadowing locals renamed where signals/methods became same-named
  (size_changed, worker_count, selected_items, import param, filters)
- third_party OFX member/namespace usages restored (OFX::Host::*,
  _created, _clipPrefsDirty, createInstance, clearPersistentMessage)
- STL protocol aliases restored (const_iterator) with .clang-tidy
  ignore rules; qHash overloads restored

Full build and test suite pass: ctest 4/4, ~1960 gtest cases green.

core/src/util/fractionutils.cpp

@@ -53,7 +53,7 @@ int64_t i64_gcd(int64_t a, int64_t b)
 
 } // namespace
 
-void ReduceFraction(int64_t &num, int64_t &den, int64_t max)
+void reduce_fraction(int64_t &num, int64_t &den, int64_t max)
 {
 	if (den == 0) {
 		num = 0;
@@ -110,7 +110,7 @@ void ReduceFraction(int64_t &num, int64_t &den, int64_t max)
 	den = a1d;
 }
 
-int CompareFractions(int an, int ad, int bn, int bd)
+int compare_fractions(int an, int ad, int bn, int bd)
 {
 	const int64_t tmp = an * int64_t(bd) - bn * int64_t(ad);
 
@@ -125,7 +125,7 @@ int CompareFractions(int an, int ad, int bn, int bd)
 	return INT_MIN;
 }
 
-int64_t RescaleRnd(int64_t a, int64_t b, int64_t c, FractionRounding rnd)
+int64_t rescale_rnd(int64_t a, int64_t b, int64_t c, FractionRounding rnd)
 {
 	// Normalize so that the divisor is positive; the sign is carried by the
 	// dividend instead.
@@ -142,7 +142,7 @@ int64_t RescaleRnd(int64_t a, int64_t b, int64_t c, FractionRounding rnd)
 	unsigned __int128 uc = static_cast<unsigned __int128>(c);
 
 	unsigned __int128 q;
-	if (rnd == FractionRounding::kNearInf) {
+	if (rnd == FractionRounding::k_near_inf) {
 		// Round to nearest, ties away from zero
 		q = (ur + uc / 2) / uc;
 	} else {