refactor: remove legacy defaults migration

[ygsgdbd]

Summary

TypeSwitch has used the Sharing-backed file store since v0.5.1, and v0.9.0 shipped the final recovery migration for affected legacy rules. Keeping the old Defaults importer indefinitely now adds startup sequencing, directory scanning, dependencies, and tests for an upgrade path the project no longer intends to support.

This change sets v0.9.0+ as the direct-upgrade compatibility baseline and:

• removes the legacy Defaults migration, reducer effects, and migration-only application scanning;
• removes migration-specific tests while preserving the current JSON/schema compatibility coverage under AppStorageCompatibilityTests;
• removes the now-unused SwifterSwift dependency and updates the English and Chinese dependency documentation;
• documents the compatibility boundary in both Unreleased changelog sections.

Existing Sharing storage paths, JSON schemas, and MigrationVersion remain unchanged. The old Defaults keys are not actively deleted from user systems.

Testing

• xcodebuild test -project TypeSwitch.xcodeproj -scheme TypeSwitch -destination 'platform=macOS' -testLanguage zh-Hans -skipPackagePluginValidation -skipMacroValidation CODE_SIGN_IDENTITY='' CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO — 131 tests, 0 failures.
• SwiftFormat 0.62.1 lint — 0 of 43 files require formatting.
• script/test_release_scripts.sh — passed.

Changelog

Select exactly one:

• Updated both Chinese and English entries under Unreleased in CHANGELOG.md.
• No changelog entry is needed. Reason: N/A

Privacy

• This change does not add telemetry or upload user data, or the reason this is not applicable is explained above.

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