serviceability: tenant + permission instruction builders (Rust)

[juan-malbeclabs]

Part of #4014.

Add src/tenant.rs and src/permission.rs builders (tenant CRUD; create/update/delete permission).

Acceptance

• One pure builder per non-deprecated tenant/permission instruction.
• Verbatim account-layout doc-comments.
• Unit tests + ix_* golden fixtures.

[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.