Add Small Seal Script dictionaries and configs for the Unicode 18.0 Seal block

[frankslin]

Unicode 18.0 encodes 11,328 Small Seal characters at U+3D000..U+3FC3F and publishes their modern CJK equivalents as the kSEAL_MCJK property of SealSources.txt (UAX #60). This change turns that data into a bidirectional OpenCC mapping between Small Seal characters and Han characters, bridges modern standard characters to the 說文 transcriptions used by kSEAL_MCJK, and adds three experimental configs: t2seal.json, seal2t.json and s2seal.json.

Detailed Changes:

• Dictionaries:
  • Add SealCharacters.txt (Small Seal -> modern CJK equivalent, 11,328 keys) derived from kSEAL_MCJK. When several Small Seal characters share one Han form, the reverse keeps code point order so the 說文解字 headword precedes its 重文; four @reverse-prefer lines promote the candidate attested in more source editions. U+3D06D (說文 玉) maps to 玉 instead of the kSEAL_MCJK value 王, which collides with U+3D068 (說文 王).
  • Add SealVariants.txt (modern standard character -> 說文 transcription, e.g. 年 -> 秊, 前 -> 歬, 小 -> 𡭔), compiled by aligning the headword sequence of the shuowen.org open data (Apache-2.0) with kSEAL_MCJK and supplemented by reviewed Unihan variant pairs and regional variants. Self-mappings with @reverse-prefer keep transcriptions that are themselves standard characters (裏, 爲) unchanged in seal2t.
  • SealCharactersRev and SealVariantsRev are produced at build time with reverse.py, like TWVariantsRev.
• Configs: t2seal = SealVariants -> SealCharactersRev; seal2t = SealCharacters -> SealVariantsRev; s2seal = the s2t chain followed by the t2seal stages. Small Seal to Simplified needs a seal2t | t2s pipeline because segmentation only runs on the input.
• Build: wire the new dictionaries into Bazel (TEXT_DICTS, reverse genrule, non-BMP audit exclusion) and CMake (DICTS_RAW, DICTS_GENERATED, CONFIG_FILES).
• CLI: list the configs in the native CLI help and the npm CLI built-in config table.
• Tests / docs: add t2seal / seal2t / s2seal cases to testcases.json; document the configs in README, node READMEs, CONTRIBUTING, AGENTS and NEWS.