serviceability-instruction: golden fixtures + CI guard (RFC-26 RF)

[juan-malbeclabs]

Summary

RFC-26 RF: golden fixtures for the instruction builders + a CI drift guard.

• Extend the serviceability fixture generator to depend on doublezero-serviceability-instruction and emit, per instruction, ix_<name>.bin (wire bytes = tag + borsh) and ix_<name>.json ({ variant, data_hex, accounts: [{pubkey, is_signer, is_writable}] }) from fixed, deterministic inputs.
• Representative set covering the trickiest layouts: create_device (variable dz_prefix), delete_device (atomic close), create_link, create_subscribe_user (optional feed), create_user (length-detected), clear_topology / assign_topology_node_segments (batched), set_global_config (config PDA + all 8 pools).
• make generate-fixtures-check (regenerate + fail on drift, scoped to the emitted .bin/.json) and a fixtures-check job in the rust workflow.

These fixtures capture byte-for-byte the current SDK trailing convention (they will drive Go/Python/TS parity in a later phase).

Testing Verification

• make generate-fixtures-check passes (regenerated output is byte-identical to the committed fixtures); a hand-edited fixture makes it fail as expected.

Closes #4026. Part of RFC-26 (rfcs/rfc26-rust-instruction-builder-library.md).

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PR stack (RFC-26 builder library)

Stacked PRs, merge in order (each based on the previous one's branch):

1. serviceability-instruction: scaffold pure instruction-builder crate (RFC-26 R0) #4049 — R0 scaffold + exemplars
2. serviceability-instruction: device domain builders (RFC-26 R1) #4050 — R1 device
3. serviceability-instruction: link domain builders (RFC-26 R2) #4051 — R2 link
4. serviceability-instruction: user domain builders (RFC-26 R3) #4052 — R3 user
5. serviceability-instruction: location/exchange/contributor builders (RFC-26 R4) #4053 — R4 location/exchange/contributor
6. serviceability-instruction: multicastgroup + allowlist builders (RFC-26 R5) #4054 — R5 multicastgroup + allowlists
7. serviceability-instruction: tenant + permission builders (RFC-26 R6) #4055 — R6 tenant + permission
8. serviceability-instruction: topology + feed builders (RFC-26 R7) #4056 — R7 topology + feed
9. serviceability-instruction: accesspass + resource builders (RFC-26 R8) #4057 — R8 accesspass + resource
10. serviceability-instruction: globalstate/globalconfig/allowlist/index/migrate builders (RFC-26 R9) #4058 — R9 globalstate/config/allowlist/index/migrate
11. this PR — RF fixtures + CI guard

R10 (commands/* migration + program-test) is the final PR and will carry the global changelog entry.