Release 2.3.2: split internal mapping definitions into utsuho.mappings

[juno-rmks]

Summary

This PR prepares 2.3.2.

The main change is an internal refactor that splits the former utsuho.maps module into a dedicated utsuho.mappings package and reorganizes mapping definitions by responsibility, without changing conversion behavior.

Changes

• add utsuho.mappings.common for shared mapping type aliases
• add utsuho.mappings.width for half-width/full-width mapping definitions
• add utsuho.mappings.kana for hiragana/katakana mapping definitions
• add utsuho.mappings as the import entry point for internal mapping definitions
• update utsuho.converters to import from utsuho.mappings
• remove the legacy internal utsuho.maps module
• add the 2.3.2 design/change document to the documentation
• update the documentation index and changelog for 2.3.2

Notes

• this is an internal refactor only
• conversion rules and conversion results are intended to remain unchanged
• utsuho.maps was treated as an internal implementation detail and removed accordingly

Verification

• local regression tests were reported as passing with the project virtual environment
• benchmark JSON for this release candidate was generated as 0003_2.3.2.json
• benchmark comparison against 0002_2.2.1.json did not show a clear performance regression

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Restore utsuho.maps compatibility or update API docs

Switching imports to utsuho.mappings here coincides with removing the utsuho.maps module, but the published API docs still reference utsuho.maps (docs/source/api/utsuho.rst:17, docs/source/api/utsuho.maps.rst:4). In this state, import utsuho.maps raises ModuleNotFoundError, so doc generation for that page and any users following the documented path break; either keep a compatibility shim module or update/remove those API doc entries in the same change.

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