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.

[ben-dz]

Two things worth changing before merge; nothing blocking. (1) The new fixtures-check job in rust.yml is a strict duplicate of the pre-existing sdk-fixture-drift job in sdk.yml — same make check-fixtures, both triggered on every PR/push with no path filters — so the drift check runs twice per PR for zero added coverage, and the new copy runs on an oversized ubuntu-24.04-16c-64gb runner with no timeout-minutes. Delete one, or move the check and delete the other. (2) The set_global_config fixture is built from SetGlobalConfigArgs::default(), emitting an all-zero body that pins only the length — it can't catch field-order/endianness/signedness/Option-encoding regressions, which is the whole point of a wire-format golden; use distinct nonzero values. Low: the fixture JSON records neither program_id nor the input args, so a future parity harness must read the Rust source to reproduce inputs. Fixtures are deterministic (regenerated locally, byte-identical) and otherwise correctly capture the builder output for the representative set.

[ben-dz]

All three findings from the prior review are addressed in ebe5e54 and verified in the current code: the duplicate fixtures-check CI job is deleted from rust.yml (sdk.yml's better-provisioned sdk-fixture-drift retained); set_global_config now builds SetGlobalConfigArgs with distinct nonzero values so the golden pins field order, both u32 ASNs, and the optional community's Some-encoding instead of an all-zero body; and IxFixtureMeta records program_id in every ix_*.json. The passing sdk-fixture-drift job confirms the goldens regenerate byte-identically. lgtm.