serviceability: topology + feed instruction builders (Rust)

[juan-malbeclabs]

Part of #4014.

Add src/topology.rs and src/feed.rs builders.

Notes

• Topology clear_topology / assign_topology_node_segments are batched: expose a single-chunk builder plus a *_batched(...) -> Vec<Instruction> convenience. Move the batch-size consts into this crate (the 32-account cap math includes the trailing accounts the builder now owns).

Acceptance

• One pure builder per non-deprecated topology/feed instruction; batched convenience helpers where applicable.
• Unit tests (incl. a batching boundary test) + 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.