docs(lore-0173): the oracle feed every USD price depends on was never justified

[karczuRF]

Raised while walking through the oracle path for 0167. The question "why this contract?" has no answer anywhere in the repo.

pub const REFLECTOR_CEX_DEX: &str = "CAFJZQWSED6YAWZU3GWRTOCNPPCGBN32L7QV43XX5LZLFTK6JLN34DLN";

A bare constant since ffc07e4, with no ADR behind it — and there are nine ADRs covering backfill strategy, PK design, the runtime framework and the CH host, but not the single external dependency the entire USD estate rests on.

⚠️ Worth stating plainly: Reflector is not an official Stellar oracle. SDF doesn't operate one. SEP-40 is a standard — the interface a price oracle should expose — and "SEP-40 compliant" reads as endorsement if you're not looking closely.

Alternatives were observed, never evaluated

0048 sampled real Soroban events and found four live feeds:

Contract	Feed	Keyed by
CALI2BYU…LE6M	Reflector — Stellar on-chain assets	token contract address
CBKGPWGK…CJZC	Reflector — FX	symbol
CAFJZQWS…4DLN	Reflector — global crypto	symbol ← chosen
CA526Y2N…XUSG	RedStone	bytes-encoded XDR

0048 records that they exist; 0039 records that we call one. Neither records a decision.

The one worth assessing

CALI2BYU…LE6M keys by Stellar token contract address, not global ticker — a materially different question. Our feed says what "USDC the ticker" trades at externally; that one would say what a specific Stellar asset is worth.

This bears on an unmarked assumption: reflector_key_to_identity asserts Reflector's USDC means USDC at the canonical Circle issuer. Almost certainly right — but 0165 measured 56 other issuers using the code USDC, about which our feed can say nothing. No test, no comment marking it as an assumption.

It may also be informative for 0172 (USDT/USDC at ~0.14 on SDEX): a Stellar-native oracle would corroborate or contradict the local market, and either answer tells us something.

This is not a proposal to switch. The current feed is plausibly right for a USD reference — you want the real-world dollar value, not a thin local market's opinion. The deliverable is a decision on the record.

Why now

0167 is about to snapshot these readings into a forever-retained table. Sanity-checking the source belongs before it becomes permanent history — and it's cheap while usd_rate is still empty and unconsumed.

Also in scope

• A policy for config that already exists with no policy behind it: REFLECTOR_CONTRACT and SOROBAN_RPC_URL are both overridable, so we have the mechanism to switch and no documented answer to when.
• Whether to poll a second feed as a cross-check — noting this isn't free: oracle_prices is keyed on oracle_name and could hold both, but 0167's usd_rate has no oracle column, so two feeds disagreeing at one timestamp needs a precedence rule first or the snapshot winner is arbitrary.
• REFLECTOR_DECIMALS = 14 is another undocumented external assumption that happens to line up exactly with our Decimal(38,14).