Design migration tooling after read-only analysis is reliable

[Zach677]

Context

Modern.UIKit should not promise automatic migration until read-only analysis is reliable across enough real repo shapes.

Scope

Define the first supported migration slice after analysis gates are strong enough. Candidate directions:

• SwiftUI entry wrapped by a UIKit shell
• Tuist workflow hardening without replacing Tuist
• UIKit/Xcode additive baseline completion
• documentation-only adoption for repos that should preserve their existing architecture

Acceptance

• Migration tooling has a narrow write scope.
• Every write path is guarded by can_apply or an equivalent explicit capability.
• Existing source of truth, signing, bundle identifiers, remotes, and product source are preserved by default.
• The feature has real fixture coverage before being advertised in README.

[linear-code]

SPA-58

[Zach677]

Parking this intentionally: the first migration slice should be shaped by a real repository that needs the migration, after #1 and #3 prove the read-only analysis reliable. No design work until that trigger fires.