Atomic swaps: fix offer publishing, harden fee/balance validation, generic Ethereum RPC endpoint, any-ERC-20 and Confidential-Asset swaps

[Maxnflaxl]

Atomic swaps have been broken since 7.3 and accumulated a set of open issues. This PR fixes the breakage, hardens the validation around swaps, and extends them to arbitrary ERC-20 tokens and Beam Confidential Assets. One commit per issue plus two small standalone commits (a stale fee-rate constant, CLI cleanups); tracking issue #2070.

Fixes

#2071 — swap offers rejected as foreign. Since d4d0e78 ("using nonce addresses") FillSwapTxParams() allocated a bare key id and never persisted a WalletAddress, but SwapOffersBoard only recognizes publishers found in getAddresses(true) — every offer threw ForeignOfferException, breaking publishing in UI, CLI and API (BeamMW/beam-ui#1017, BeamMW/beam-ui#1073). The publisher address is now created and saved with Auto expiration; a board regression test drives the real FillSwapTxParams → publishOffer path.

#1697 — CLI allowed swaps exceeding the swap-coin balance. swap_init and the accept path now check the live side-chain balance (ether/tokens/BTC family) before starting.

#1039 — lock-tx fee rate unverified after funding. The pre-flight validator assumes a fixed-size tx, so many small UTXOs could produce an unconfirmable lock tx. fundRawTransaction now surfaces the actual fee (bitcoin-core returns it; Electrum computes it during coin selection) and BitcoinSide fails the LOCK_TX subtransaction with FeeIsTooSmall when fee/vsize is below the configured rate. The acceptance bound tops each unsigned input up from its 41-byte serialized skeleton to the 68-vbyte signed P2WPKH lower bound, so segwit-funded transactions cannot false-positive.

Stale 15 gwei mainnet fee-rate floor. Mainnet gas has been well below 15 gwei since the merge, so the floor made EstimateGasPrice discard every real node suggestion (recommended rate 0, surfaced as a connection error) and rejected any manually entered fee below 15 gwei. The floor is now 1 gwei and the recommendation is clamped up to it instead of zeroed.

#2073 — inverted kernel-validity check in fly_client.cpp. Found while resurrecting swap_test: the fetched-kernel path of ProofKernel2 handling threw ThrowUnexpected when the kernel IS valid (refactor regression from 7a38b48), turning every swap redeem into an endless reconnect loop. One-character fix restoring the pre-refactor semantics; without it the first full-swap test scenario hangs, with it the suite passes.

Features

#2072 — generic Ethereum JSON-RPC endpoint. The bridge hardcoded Infura's URL shape. Settings gain a custom-RPC mode with a strictly validated endpoint URL (http/https only, hostname or IPv4, port 1–65535, no embedded credentials; bracketed IPv6 literals are rejected because the HTTP transport is IPv4-only and would silently never connect; endpoint URLs never reach logs unsanitized since provider paths carry API keys). New getChainID RPC plus Client::ValidateEndpoint() report chain id and latest block before a swap is attempted; mainnet builds reject non-mainnet chains. Existing stored settings load unchanged as Infura mode.

BeamMW/beam-ui#1166 — any ERC-20 token and any Confidential Asset. - Wire format: new AtomicSwapCoin::Erc20Token and an ExtendedOffer marker appended at the old Unknown ordinal, plus tx parameters 41–45 (beam asset id/name, token contract/symbol/decimals), all frozen by static_asserts. Extended offers broadcast with the ExtendedOffer wire coin so pre-change wallets drop them via their existing m_coin >= Unknown guard; classic offers stay byte-identical (locked by a golden-vector test). - ERC-20 side: per-offer token contract; symbol()/decimals() read via eth_call (ABI-string decode with bytes32 fallback, exposed through Client::GetTokenInfo) and always shown for explicit confirmation; decimals bounded (kMaxTokenDecimals = 18) at the board, the bridge decode and the unit helpers to prevent amount mis-scaling; contract and token parameters validated by one shared helper (IsValidEthContractAddress/GetValidatedErc20Params) in the CLI, the API and the offers board; CLI swap_coin=erc20 --token_contract and API mirror. The Erc20Token second side registers the Ethereum factory, and the board's incomplete-offer recovery re-broadcasts a terminal status with the full parameter set (a parameterless extended-offer update would be dropped by every peer). - Confidential-Asset side: an asset can take the Beam side of the HTLC. The shared output's commitment is built on the asset generator; the cosigned rangeproof runs over its blinded form (CoSign with pHGen); the surjection proof is derived by both parties from a generator-blinding scalar hashed from the shared bulletproof seed, so neither party trusts the other about the asset id (this mirrors core's own Output::Create MPC construction — no consensus changes). Refund/redeem withdraw the full asset amount; the kernel fee stays BEAM, funded from the tx owner's own coins, so the asset redeemer must hold BEAM for the redeem fee (validated and surfaced in CLI/API). - Plumbing fixes surfaced by asset withdraws: AddSharedInput no longer drops the shared input when owner-side inputs exist, and completed-swap coin statuses are applied per sub-transaction (with the unused refund/redeem reservation released) instead of txID-wide.

Testing

• swap_test resurrected as the e2e vehicle (disabled since 2023; wallet/unittests/CMakeLists.txt). Full suite green in ~3.5 min: classic swaps (4 directions), CA swaps in both directions against a real mining node (the shared asset output passed actual consensus validation), CA refund, BTC/Beam refunds, cancel/expire matrix, electrum variants. Two scenarios (TestSwapTransactionWithoutChange, TestIgnoringThirdPeer) remain disabled: pre-fork3 a kernel proof verifies only while its block is the tip, and the instant-advancing TestNode always loses that race — a test-environment limitation with no mainnet relevance (mainnet is past fork3).
• swap_board_test extended: publisher-address regression, raw-wire-coin rejection, decimals bounds, wire-stability golden vector, extended-offer round trip with on-wire snooping.
• New eth_rpc_endpoint_test (URL parser accept/reject matrix, settings round-trip, decimals-word parsing, token-param tokenization round trip); bitcoin_rpc_test extended for the funded-fee bound; wallet_api_test covers the token pre-flight balance decision helper.

Notes for review

• Wire-compat invariant: classic offer serialization is byte-identical (golden test); old wallets drop extended offers silently.
• Known limitation: an out-of-band token (not board offer) for a Confidential-Asset swap is accepted by legacy wallets as a plain-BEAM swap of the same numeric amount — the asset travels in parameters legacy clients store but ignore, and the token format has no version gate. Negotiation then fails safely (the co-signed lock output's proofs cannot be built across mismatched asset generators; the tx rolls back before anything locks on the foreign chain), but with a generic multisig error rather than an up-front rejection.
• Offer previews reuse one saved publisher address per wallet session (rotated when an offer is actually published), so regenerating the preview does not grow the address book.
• The API's ERC-20 pre-flight balance check uses a per-contract cache (populated on first use + the existing refresh loop) because the v6 handlers are synchronous; a first-ever contract passes pre-flight and is enforced at lock-tx build. Watched contracts are evicted after 10 idle minutes so the poll set cannot grow without bound.
• BeamMW/beam-ui PR (branch issue/1267) depends on this branch's head via its submodule pointer — merge/push this PR first.

Closes #2071, #1697, #1039, #2072, #2073. Core half of BeamMW/beam-ui#1166. Part of #2070.