Support large-account commits through the v2 committor flow

@snawaz is already working on this.

Since Sep 14, 2026.

## Goal

Support large-account commits through the v2 committor flow defined by [delegation-program#191](https://github.com/magicblock-labs/delegation-program/issues/191).

## Why

Rejecting oversized accounts avoids a late allocation failure but leaves those accounts unsupported. A large account that already exists on base and an update that grows the destination by more than the runtime allocation limit are different cases; uploading a large state buffer alone does not solve both.

## Scope

Own MBV's adoption of the protocol flow: update scheduling restrictions and delivery preparation once v2 supports the complete upload, commit, and finalize path. Preserve account, commit-ID, and authority binding throughout preparation.

Cover large same-size updates, shrinking updates, and growth across the allocation boundary, preserving the intended bytes, lamports, and rent after finalization. Protocol support belongs to delegation-program#191; this issue does not prescribe the old dedicated-realloc-instruction proposal or include a v1 backport.