[ME][2/7] Share import transactions and simplify persistence

[tomTom1010EEE]

Description

• Share Texture2D/Cubemap import transactions between Chara and Studio, keeping commit and rollback logic together.
• Organize persistence loading and restoration into focused steps.
• Keep save fields explicit, with a local function handling serialization and empty-list values. No separate serialization helper class.

Motivation and Context

Replaces the closed #406 and follows the merged #405. This is the complete second stage in one commit, without later maintenance or UI changes. Public APIs, save formats and application order are preserved.

How Has This Been Tested?

Rebuilt API, KK, KKS, EC, AI, HS2 and PH successfully. Verified save field names, order and null handling. Existing warnings remain; this revised stage has not been tested in game.

Screenshots (if appropriate):

No visual changes.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

[starstormhun]

A lot of moving things around, nothing revolutionary.