feat(sdk#375): add the sdk_parity_fixtures binary the Parity Gate needs

[dcccrypto]

The Parity Gate's percolator-nft target has failed with cargo run failed on every run since at least 2026-08-23, because this binary did not exist. Three of the gate's four targets were in that state — so ABI drift in prog, stake and nft had no automated detection at all. Every drift defect found in that window (sdk#376, sdk#379, nft#182) was found by hand.

It immediately found a drift

That is the argument for having it, so leading with it:

	committed spec	actual struct
field at offset 167	reserved	last_holder
offset	167	167
total length	199	199

#138 renamed _reserved → last_holder in place. Nothing broke, because the SDK reads by offset — and nothing flagged it, because the gate could not run. Every other offset matches the struct exactly, so the mirror is correct and only the name drifted.

Benign, and that is precisely what makes it a good first catch: a naming drift survives indefinitely without a gate, then misleads whoever next reads the spec to work out what a byte range means. The SDK already exposes this as lastHolder (percolator-sdk#384) — the spec was the last thing still calling it reserved.

Why offset_of! and not a literal table

A hand-written offset list would restate the numbers the spec already holds, so the check would compare a copy against a copy and pass regardless of what the program does.

That is the same failure mode as this crate's own const_assert!s (see #160), and as the stale CYCLE_CAP copy found in percolator-keeper today. If PositionNftV16 changes, these values move and the gate goes red.

No new dependency

This crate builds an on-chain program. The fixture shape is small and fixed, so the JSON is emitted directly rather than adding serde_json to the program's dependency graph for a dev-only binary.

Verification

• cargo run --quiet --bin sdk_parity_fixtures produces the full spec shape
• diffed against the committed spec: one key renamed, zero offsets changed, length unchanged
• nft suite: 11 + 6 + 5 + 5 + 4 passing, 0 failed; cargo fmt --check clean

The spec file itself lives in percolator-sdk and is updated there.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NgoNgagkvw7i5SSRC3FJ8D