serviceability: globalstate/globalconfig/allowlist/index/migrate instruction builders (Rust)

[juan-malbeclabs]

Part of #4014.

Add the admin/misc builders: src/globalstate.rs, src/globalconfig.rs, src/allowlist.rs, src/index.rs, src/migrate.rs (init/set-authority/set-config/feature-flags, allowlist toggles, index, migrate).

Excluded variants (documented in lib.rs)

The enum has 116 variants (tags 0–115, contiguous); builders cover all buildable variants (~94). Excluded, with no builder emitted (no Err/panic stubs — builders are infallible):

• Explicit placeholders kept only for discriminant stability: Deprecated95, Deprecated96, Deprecated102, Deprecated103, Deprecated111.
• Deprecated handlers returning DoubleZeroError::Deprecated (e.g. ActivateDevice, RejectDevice, CloseAccountDevice, the corresponding link/user/multicast lifecycle variants, and several *DeviceInterface variants).

Acceptance

• One pure builder per non-deprecated instruction in these domains; empty-arg variants pack to just the tag byte.
• Verbatim account-layout doc-comments.
• Unit tests + ix_* golden fixtures. After this PR all buildable variants are covered.

[juan-malbeclabs]

Closing for now — this work is captured in RFC-26 (rfcs/rfc26-rust-instruction-builder-library.md). Will reopen once the RFC is approved.

[juan-malbeclabs]

Reopening: RFC-26 was merged (#4035). Reviewed this issue against the final RFC — scope and design decisions are still aligned. The implementation work tracked here is still pending.