Fix missing ICU UTF-16 header includes, C++17/20 CTAD guides, and empty INFLECTION_ROOT path resolution

[nciric]

Overview

This issue addresses compiler compatibility and runtime path resolution improvements across inflection:

1. Explicit <unicode/utf16.h> includes:
   Several C++ source files use ICU UTF-16 code point macros (U16_PREV, U16_GET, U16_LENGTH, U16_NEXT) without explicitly including <unicode/utf16.h>. On strict compiler toolchains that do not transitively include ICU sub-headers, compilation fails.

2. C++17/20 Class Template Argument Deduction (CTAD) Guides:
   Template classes (Finally, CompressedArray, MarisaTrie) and test code (DialogThreadSafetyTest.cpp) rely on template argument deduction. On toolchains compiling with strict C++17/20 warning options (such as -Wctad-maybe-unsupported), compiler warnings or errors are triggered. Adding explicit deduction guides for these classes and specifying explicit template arguments (std::atomic<bool>, std::barrier<>) ensures clean compilation.

3. ResourceLocator.cpp Relative Path Resolution:
   In ResourceLocator::getRootForLocale(), when the INFLECTION_ROOT environment variable is unset/empty, string concatenation INFLECTION_ROOT_DIRECTORY() + u"/" + U16(INFLECTION_DATA_ROOT_DIR) + u"/inflection" evaluates with an unintended leading slash (u"/..."). This converts relative INFLECTION_DATA_ROOT_DIR paths into absolute filesystem root paths. Updating getRootForLocale to check if INFLECTION_ROOT_DIRECTORY() is empty preserves intended relative directory pathing.

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Proposed Changes

• Header Includes: Add #include <unicode/utf16.h> to:

  • src/inflection/tokenizer/ControlCleaver.cpp
  • src/inflection/grammar/synthesis/HeGrammarSynthesizer_WithConditionalHyphen.cpp
  • src/inflection/grammar/synthesis/KoGrammarSynthesizer_ParticleResolver.cpp
  • src/inflection/grammar/synthesis/TrGrammarSynthesizer_TrDisplayFunction.cpp
  • src/inflection/lang/StringFilterUtil.cpp
  • src/inflection/tokenizer/TokenUtil.cpp
  • src/inflection/util/StringViewUtils.cpp
  • src/inflection/util/UnicodeSetUtils.cpp

• CTAD Guides & Test Updates:

  • src/inflection/util/Finally.hpp: Add deduction guide template <typename Function> Finally(Function) -> Finally<Function>;
  • src/inflection/dictionary/metadata/CompressedArray.hpp: Add deduction guide template <typename T> CompressedArray(const std::vector<T>&) -> CompressedArray<T>;
  • src/inflection/dictionary/metadata/MarisaTrie.hpp: Add deduction guide template <typename T> MarisaTrie(const std::map<std::u16string_view, T>&) -> MarisaTrie<T>;
  • test/src/inflection/dialog/DialogThreadSafetyTest.cpp: Specify explicit template parameters for std::atomic<bool> and std::barrier<>.

• Path Resolution:

  • src/inflection/util/ResourceLocator.cpp: Check if INFLECTION_ROOT_DIRECTORY() is empty before prepending u"/".