serviceability: location + exchange + contributor instruction builders (Rust)

[juan-malbeclabs]

Part of #4014.

Add src/location.rs, src/exchange.rs, src/contributor.rs builders — the simple account_index-seeded CRUD template (create/update/suspend/resume/delete), single authorize() gate.

Acceptance

• One pure builder per non-deprecated instruction in these three domains.
• 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.