serviceability: link instruction builders (Rust)

[juan-malbeclabs]

Part of #4014.

Add src/link.rs builders for the full link domain (create/accept/update/suspend/resume/delete, link health). Covers the split_trailing_permission family.

Notes

• split_trailing_permission builders derive the Permission PDA from the payer and append it read-only as the last account (never a caller-supplied parameter). This append ships commented out until the instruction's authorize() migration is activated. It remains safe once enabled because the processor peels the trailing Permission account by PDA match.

Acceptance

• One pure builder per non-deprecated link 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.