feat(rwa-yield-accuracy): bench 089 scaffold + USDY rebase probe (Sprint 1/3)

[Flotapponnier]

First sprint of the killer signature bench that measures on-chain delivered yield vs advertised APY for the top 5 tokenized U.S. Treasuries.

What ships now (Sprint 1)

• Full spec YAML with 5 providers declared (BUIDL, BENJI, USDY, USTB, OUSG)
• Harness scaffold : config, metrics, loghub, promised-yield loader, IssuerProbe interface
• USDY probe fully implemented (rebase model, simplest)
• promised-yields.yml with all 5 tokens (curated APY + source URLs + dates)

Deferred to Sprint 2 (next PR)

• BUIDL + USTB probes (dividend model : sum USDC Transfer events from treasury wallet)
• BENJI + OUSG probes (NAV appreciation model : fetch daily NAV from issuer + cross-check on-chain oracle)

Deferred to Sprint 3

• Deploy to VPS ocb-par-main
• Cross-check each measurement against issuer's own fund fact sheets
• Content : whitepaper draft + launch tweet + press outreach

Design decisions

• V1 = Ethereum only for all 5 tokens. Multi-chain (BUIDL 6 chains, USDY 4 chains) in V2.
• Rebase model measured via totalSupply(t=now) vs totalSupply(t=window ago). Block-by-timestamp uses linear approximation (post-merge 12s block time), accurate to ~10 blocks over 30d = well below yield noise floor.
• Dividend model will use eth_getLogs on USDC Transfer events filtered by from=treasury_wallet, summed over the window.
• NAV model relies on issuer's public daily NAV endpoint, cross-checked with the smart contract's own price oracle (Chainlink PoR for BUIDL, native oracle for BENJI).
• Promised APY hot-reloaded from promised-yields.yml every 60s so a manual weekly edit lands within one scrape cycle.

Metrics contract (Prometheus)

rwa_yield_promised_bps{issuer, token, chain}
rwa_yield_delivered_bps_30d / _7d / _lifetime
rwa_yield_deviation_bps_30d / _7d / _lifetime
rwa_yield_total_supply_units
rwa_yield_aum_usd
rwa_yield_probe_ok
rwa_yield_last_measured_unix
rwa_yield_probe_errors_total{error_type}
rwa_yield_distributions_usd_total

Test plan

• bun run validate — 76 specs valid (was 75)
• go build in harness — clean
• go vet — clean
• Local run against public RPC (defer until Sprint 2 when other probes shipped)
• Deploy to VPS + Prom scrape config (Sprint 3)
• Cross-check USDY 30d delivered vs Ondo dashboard (Sprint 3)