feat(go): add x402 exact (client+server)

.gitignore

@@ -34,3 +34,5 @@ harness/go-client/go-client
 mpp-sdk-self-learning/
 .build/
 go/coverage.out
+notes/codex-review-*.md
+notes/codex-review/

go/x402/README.md

@@ -0,0 +1,81 @@
+# Go x402 SDK
+
+Go implementation of the x402 `exact` scheme (client + server) for Solana.
+
+This sub-package mirrors the canonical Rust spine at `rust/crates/x402/`
+and ships the interop adapters used by the cross-language harness.
+
+## Layout
+
+```text
+go/x402/
+├── cmd/
+│   ├── interop-client/    interop harness client binary
+│   └── interop-server/    interop harness server binary
+└── README.md
+```
+
+The exact-scheme protocol types, verifier, and settler live inline in
+the two `main.go` files. The Rust crate keeps a separate
+`protocol/schemes/exact/`, `server/exact.rs`, `client/exact/payment.rs`
+split; the Go port keeps them inline because both binaries are
+self-contained and there is no third caller. The spine's wire format,
+constants, and pipeline ordering are mirrored 1:1.
+
+## Test
+
+```bash
+cd go
+go test ./x402/... -cover -race
+```
+
+Expected coverage: server ≥ 90 %, client ≥ 90 %.
+
+## Format and vet
+
+```bash
+gofmt -l go/x402/
+go vet ./x402/...
+```
+
+## Parity with the Rust spine
+
+The Go port matches `rust/crates/x402/` on:
+
+- CAIP-2 network identifiers (`solana:5eykt...`, `solana:EtWTR...`,
+  `solana:4uhc...`) — verbatim.
+- Program IDs (Token, Token-2022, Associated Token, Compute Budget,
+  System, Memo, Lighthouse) — verbatim.
+- Stablecoin mint addresses per network (USDC/USDT/USDG/PYUSD/CASH) —
+  verbatim.
+- Constants: `EXACT_SCHEME = "exact"`, `maxMemoBytes = 256`.
+- Instruction allowlist: ComputeBudget (Set CU Limit + Price), SPL
+  Token / Token-2022 `TransferChecked`, plus optional Lighthouse +
+  Memo + ATA-create.
+- Lighthouse passthrough by program-ID match only (no discriminator
+  allowlist, no account-count cap) — spine parity.
+- Fee-payer-in-instruction-accounts sweep with the legitimate
+  ATA-create payer slot exception.
+- Destination ATA re-derived from `(payTo, mint, tokenProgram)` and
+  compared against the transaction's destination index.
+- L8 settlement ordering: broadcast → confirm → mark.
+- Cross-server credential rejection with canonical 4xx + token in body.
+- Env-var contract: `X402_INTEROP_TARGET_URL`,
+  `X402_INTEROP_RPC_URL`, `X402_INTEROP_NETWORK`,
+  `X402_INTEROP_CLIENT_SECRET_KEY`,
+  `X402_INTEROP_FACILITATOR_SECRET_KEY`, `X402_INTEROP_PAY_TO`,
+  `X402_INTEROP_MINT`, `X402_INTEROP_PRICE`,
+  `X402_INTEROP_EXTRA_OFFERED_MINTS` (CSV),
+  `X402_INTEROP_PREFER_CURRENCIES` (CSV).
+- Client `result` and server `ready` stdout JSON shapes.
+
+Intentional Go-side specifics (not divergences):
+
+- Mint alias resolution happens at the env-read boundary
+  (`X402_INTEROP_MINT` may be a symbol or base58); the rest of the
+  code sees canonical base58. The spine accepts the same pattern.
+- Duplicate-settlement cache keys are SHA-256 of the encoded
+  transaction, in addition to Solana's native per-signature
+  uniqueness — defense-in-depth, matches the upstream reference.
+
+No upstream behavior changes vs the reference port (tip `e3bf746`).