specs(exact): propose TON exact scheme for x402 v2 (spec-only)

specs/schemes/exact/scheme_exact.md

@@ -30,4 +30,12 @@ While implementation details vary by network, facilitators MUST enforce security
 - Transfer correctness: `to` MUST equal `payTo` and `amount` MUST equal `requirements.amount` exactly.
 - Simulation verification: MUST emit events showing only the expected balance changes (recipient increase, payer decrease) for `requirements.amount`—no other balance changes allowed.
 
-Network-specific rules are in per-network documents: `scheme_exact_svm.md` (Solana), `scheme_exact_stellar.md` (Stellar), `scheme_exact_evm.md` (EVM), `scheme_exact_sui.md` (SUI).
+### TON
+
+- Transfer correctness: exactly 1 `jetton_transfer` with destination equal to `payTo` and amount equal to `requirements.amount` exactly.
+- Signature validity: Ed25519 signature MUST verify against a public key derived from the BoC's `stateInit` (seqno == 0) or from the on-chain `get_public_key` getter (seqno > 0). Only `internal_signed` (0x73696e74) opcode is supported in the current gasless flow.
+- Wallet code validity: contract code MUST match a known W5 wallet contract, using `stateInit` for `nonexist`/`uninit` wallets and on-chain code for `active` wallets.
+- Replay protection: seqno MUST be strictly equal to on-chain value; duplicate `settlementBoc` submissions rejected via BoC hash dedup.
+- Simulation verification: SHOULD simulate via emulation during `/verify` to confirm expected balance changes.
+
+Network-specific rules are in per-network documents: `scheme_exact_svm.md` (Solana), `scheme_exact_stellar.md` (Stellar), `scheme_exact_evm.md` (EVM), `scheme_exact_sui.md` (SUI), `scheme_exact_ton.md` (TON).