feat: electrum client (webpki TLS, bech32, checked sats)

BTC/LTC Electrum/Fulcrum client, reworked clean from the chain-audit
playbook (not ported). Every byte from a server is treated as hostile.

Security:
- TLS verified against the webpki root store WITH hostname checking; the
  accept-any-certificate verifier is gone (closes the MITM hole). The
  connector is built once and reused.
- Addresses decoded via the `bech32` crate (full checksum + bech32m +
  witness version) and base58check (checksum); the hand-rolled bech32
  underflow panic is removed. Coin/network enforced (reject ltc-on-btc,
  testnet-on-mainnet) — no byte-indexing of untrusted input.
- Amounts pass a fallible checked f64->sat converter (rejects
  non-finite/negative/>2^53) with checked_add summation; no silent
  saturation to 0 / u64::MAX. estimatefee requires finite, clamps.
- Bounded response read + per-exchange timeout (no OOM, no slowloris).
- JSON-RPC response id correlated with the request.
- Untrusted upstream text is control-stripped + length-capped before it
  reaches a (privately-logged) NodeError.

Single `call` primitive centralizes failover + id + envelope mapping;
public methods (balance/utxos/history/tip/verbose-tx/broadcast/fee) are
thin wrappers. Tip-attribution helpers dropped; total_sent_from now
declines (None) without resolved prevouts instead of mislabeling a
receive-with-change as a send.

20 L2 tests: address->script KATs (P2WPKH/P2WSH/taproot/legacy), network
rejection, panic-safety on short/garbage input, checked-sat bounds,
envelope id correlation, bounded/oversized read, duplex round-trip.

Cargo.lock

@@ -300,13 +300,19 @@ version = "0.10.0-beta"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "98f7eed2b2781a6f0b5c903471d48e15f56fb4e1165df8a9a2337fd1a59d45ea"
 
+[[package]]
+name = "bech32"
+version = "0.12.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "efbd3e1070bbdf4cd88a75264e18e8a26f7cb5c6949eadf0ceb85fb159cf08f8"
+
 [[package]]
 name = "bitcoin"
 version = "0.31.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "69197dee21fe23b45f5239bf88086efaa0cb8679f3e704906eb818e8ea169c14"
 dependencies = [
- "bech32",
+ "bech32 0.10.0-beta",
  "bitcoin-internals",
  "bitcoin_hashes",
  "hex-conservative",
@@ -2799,6 +2805,7 @@ dependencies = [
  "axum",
  "axum-extra",
  "base64 0.22.1",
+ "bech32 0.12.0",
  "bitcoincore-rpc",
  "bs58",
  "chrono",

Cargo.toml

@@ -103,6 +103,7 @@ async-trait = "0.1"
 
 # Validation
 validator = { version = "0.16", features = ["derive"] }
+bech32 = "0.12.0"
 
 [dev-dependencies]
 tokio-test = "0.4"

src/infra/mod.rs

@@ -2,3 +2,4 @@
 //! light-wallet servers). Chain clients are wired in as they land.
 
 pub mod db;
+pub mod electrum;