[frankslin]

Introduce a librime-style word-level candidate query that enumerates every form a single word can take through a converter's conversion chain (e.g. s2t expands 里 to 里 and 裏). The API is gated by OPENCC_ENABLE_UNSTABLE_API so it stays out of the installed headers and the OPENCC_ABI_VERSION contract until it is promoted at the next ABI bump.

The member is deliberately non-virtual for now, so enabling the macro does not alter the Converter vtable and remains ABI-safe; it delegates to a free function kept in a private (non-installed) header, making the eventual promotion to a virtual method a trivial change.

Detailed Changes:

• New candidate API:
  • Add GetAllConversions(const Converter&, std::string_view) in the private ConversionCandidates.hpp/.cpp, porting librime's ConvertWord loop onto OpenCC's Dict/DictEntry primitives (exact match yields all values; otherwise longest-prefix default conversion carries partial matches down the chain).
  • Add the guarded, non-virtual Converter::GetConversionCandidates member that forwards to the free function; define it alongside the free function to avoid a Bazel dependency cycle with converter_lib.
• Build wiring:
  • CMake: register the new source/private header/test and define OPENCC_ENABLE_UNSTABLE_API as PRIVATE on libopencc and the test target.
  • Bazel: add conversion_candidates_lib and its test using non-propagating local_defines, keeping the header out of exports_files.
• Tests:
  • Add ConversionCandidatesTest covering multi-value expansion, chained branch carry-through, dedup, not-found and prefix-only empties, partial conversion in a later dict, PipelineConverter returning empty, and member/free-function agreement.