Add common module management interface for labelers (add/rename/remove/duplicate subprojects)

[sdercolin]

Closes the Trello ticket Commonized module management (design discussion recorded as a comment on the card).

What

Adds a common, labeler-driven interface for managing modules (subprojects) from the main menu, replacing the need for labeler-specific macro plugins (e.g. utau-add-folder, textgrid-manage-module).

Interface (LabelerConf.moduleManagement, serialVersion 2 → 3)

"moduleManagement": {
  "add":       { "parameters": [...], "scripts": "addModule.js" },
  "rename":    { "parameters": [...], "scripts": "renameModule.js" },
  "remove":    { "scripts": "removeModule.js" },
  "duplicate": null
}

• No default implementation: module semantics differ per labeler (e.g. vlabeler-textgrid encodes the tier index/name inside the module name and its writer parses it), so an operation that is not declared is not available, and the whole File > Manage Subprojects submenu is hidden when nothing is declared. Existing labelers are unaffected.
• Each operation optionally declares displayedName, description, and plugin-style parameters, rendered by the existing plugin dialog machinery (BasePlugin / BasePluginDialogState).
• Scripts run in the Project-scope macro environment (modules, currentModuleIndex, params, labelerParams, resources, projectRootDirectory, error(), report()), targeting the current module. Results go through validate() and the undo/redo history like macro plugins.

Core fixes required by variable module counts

• ProjectHistory.contentEquals zipped module lists under the assumption // modules size will not change — removing trailing modules was treated as an index-only change and never recorded. Now a size change is never squashed.
• ProjectStoreImpl.withCurrentIndexesKept matched modules by position across history snapshots; when the module count differs it now matches by name (behavior unchanged for the normal fixed-count path).

Bundled labeler adoption

nnsvs-singer-labeler, diffsinger-labeler, and utau-singer-labeler now declare add/rename/remove (duplicate intentionally omitted — duplicating a module sharing the same raw file is unsafe with Entry-scope writers). The UTAU add script is a port of the utau-add-folder plugin, which is kept for users on older app versions. vlabeler-textgrid will adopt the interface (including scripted rename validation and duplicate) in a separate PR in its own repo.

UI

• File > Manage Subprojects submenu with one item per declared operation (hidden otherwise), with key actions available for keymap customization.
• New strings in en/zh-CN/ja/ko; READMEs (×4) updated.

Docs

• docs/labeler-development.md: new "Module Management" definition section and "Managing Modules" scripting section.
• docs/labeler-structure-updates.md: serialVersion 3.

Tests

• ModuleOperationTest: runs the real nnsvs scripts — rename (incl. collision/empty errors), remove (incl. last-module guard, index adjustment), add (lab parsed / default entry / outside-folder and duplicate errors), plus serialization round trip.
• UtauSingerProjectFixtureTest: add-folder operation end to end.
• ProjectHistoryTest / ProjectStoreTest: history recording and undo/redo across module-count changes.

Compatibility notes

• Older release builds ignore the unknown moduleManagement key (ignoreUnknownKeys), so serialVersion-3 labelers and projects saved by this version load fine there (without the feature). Nothing rejects a higher serialVersion on load.
• Project file version is unchanged (still 4).

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