constants: add BitcoinPurple (BTCP) and BitcoinPurpleTestnet network classes

Add per-chain PoW fields to AbstractNet (DIFFICULTY_ADJUSTMENT_INTERVAL,
POW_TARGET_TIMESPAN, MAX_TARGET, MAX_ADJUSTMENT_FACTOR, POW_GENESIS_BITS)
with Bitcoin defaults so existing chains are unaffected.

Add BitcoinPurple and BitcoinPurpleTestnet as independent AbstractNet subclasses
with BTCP-specific parameters:
- 120-block retarget interval, 7200-second target timespan
- powLimit 0x1e0fffff, genesis nBits 0x1e0ffff0 (POW_GENESIS_BITS)
- P2PKH prefix 56 (0x38), P2SH 55 (0x37), WIF 0xb7, bech32 HRP "btcp"/"tbtcp"
- SLIP-0132 HD key headers identical to Bitcoin mainnet/testnet respectively
- ElectrumX default ports 50001/50002 (mainnet) and 60001/60002 (testnet)

Add chain data directories:
- electrum/chains/bitcoinpurple/{servers,checkpoints,fallback_lnnodes}.json
- electrum/chains/bitcoinpurple_testnet/{servers,checkpoints,fallback_lnnodes}.json
servers.json is populated with 5 known BTCP ElectrumX nodes (TCP 51001, SSL 51002)

electrum/chains/bitcoinpurple/checkpoints.json

@@ -0,0 +1 @@
+[]

electrum/chains/bitcoinpurple/fallback_lnnodes.json

@@ -0,0 +1 @@
+{}

electrum/chains/bitcoinpurple/servers.json

@@ -0,0 +1,32 @@
+{
+    "173.212.224.67": {
+        "pruning": "-",
+        "s": "51002",
+        "t": "51001",
+        "version": "1.4.2"
+    },
+    "144.91.120.225": {
+        "pruning": "-",
+        "s": "51002",
+        "t": "51001",
+        "version": "1.4.2"
+    },
+    "66.94.115.80": {
+        "pruning": "-",
+        "s": "51002",
+        "t": "51001",
+        "version": "1.4.2"
+    },
+    "89.117.149.130": {
+        "pruning": "-",
+        "s": "51002",
+        "t": "51001",
+        "version": "1.4.2"
+    },
+    "84.247.169.248": {
+        "pruning": "-",
+        "s": "51002",
+        "t": "51001",
+        "version": "1.4.2"
+    }
+}

electrum/chains/bitcoinpurple_testnet/checkpoints.json

@@ -0,0 +1 @@
+[]

electrum/chains/bitcoinpurple_testnet/fallback_lnnodes.json

@@ -0,0 +1 @@
+{}

electrum/chains/bitcoinpurple_testnet/servers.json

@@ -0,0 +1 @@
+{}

electrum/constants.py

@@ -81,6 +81,16 @@ class AbstractNet:
     XPUB_HEADERS: Mapping[str, int]
     XPUB_HEADERS_INV: Mapping[int, str]
 
+    # PoW difficulty parameters (Bitcoin defaults; override per chain as needed)
+    DIFFICULTY_ADJUSTMENT_INTERVAL: int = 2016   # blocks per retarget window
+    POW_TARGET_TIMESPAN: int = 14 * 24 * 60 * 60  # target seconds per window
+    MAX_TARGET: int = 0x00000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffff  # compact 0x1d00ffff
+    MAX_ADJUSTMENT_FACTOR: int = 4
+    # When genesis nBits != target_to_bits(MAX_TARGET), set this to the genesis
+    # nBits value so get_target(-1) returns the correct initial difficulty.
+    # None means "derive from MAX_TARGET" (correct for Bitcoin where they match).
+    POW_GENESIS_BITS: Optional[int] = None
+
     @classmethod
     def max_checkpoint(cls) -> int:
         return max(0, len(cls.CHECKPOINTS) * 2016 - 1)
@@ -259,6 +269,81 @@ class BitcoinMutinynet(BitcoinTestnet):
     LN_DNS_SEEDS = []
 
 
+class BitcoinPurple(AbstractNet):
+
+    NET_NAME = "bitcoinpurple"
+    TESTNET = False
+    WIF_PREFIX = 0xb7
+    ADDRTYPE_P2PKH = 56
+    ADDRTYPE_P2SH = 55
+    SEGWIT_HRP = "btcp"
+    BOLT11_HRP = SEGWIT_HRP
+    GENESIS = "000003823fbf82ea4906cbe214617ce7a70a5da29c19ecb1d65618bcf04ec015"
+    DEFAULT_PORTS = {'t': '50001', 's': '50002'}
+    BLOCK_HEIGHT_FIRST_LIGHTNING_CHANNELS = 0
+
+    XPRV_HEADERS = {
+        'standard':    0x0488ade4,  # xprv
+        'p2wpkh-p2sh': 0x049d7878,  # yprv
+        'p2wsh-p2sh':  0x0295b005,  # Yprv
+        'p2wpkh':      0x04b2430c,  # zprv
+        'p2wsh':       0x02aa7a99,  # Zprv
+    }
+    XPRV_HEADERS_INV = inv_dict(XPRV_HEADERS)
+    XPUB_HEADERS = {
+        'standard':    0x0488b21e,  # xpub
+        'p2wpkh-p2sh': 0x049d7cb2,  # ypub
+        'p2wsh-p2sh':  0x0295b43f,  # Ypub
+        'p2wpkh':      0x04b24746,  # zpub
+        'p2wsh':       0x02aa7ed3,  # Zpub
+    }
+    XPUB_HEADERS_INV = inv_dict(XPUB_HEADERS)
+
+    # Provisional BIP44 coin type (not SLIP-0044 registered; matches BTCP P2P port)
+    BIP44_COIN_TYPE = 13496
+    LN_REALM_BYTE = 0
+    LN_DNS_SEEDS = []
+
+    # BTCP retargets every 120 blocks with a 7200-second target timespan
+    DIFFICULTY_ADJUSTMENT_INTERVAL = 120
+    POW_TARGET_TIMESPAN = 7200  # 120 * 60 seconds
+    # powLimit: absolute upper bound for any target (compact 0x1e0fffff)
+    MAX_TARGET = 0x00000fffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
+    MAX_ADJUSTMENT_FACTOR = 4
+    # Genesis nBits is 0x1e0ffff0, which is stricter than powLimit (0x1e0fffff).
+    # Period-0 blocks must carry this exact bits value, so we record it here.
+    POW_GENESIS_BITS = 0x1e0ffff0
+
+
+class BitcoinPurpleTestnet(BitcoinPurple):
+
+    NET_NAME = "bitcoinpurple_testnet"
+    TESTNET = True
+    SEGWIT_HRP = "tbtcp"
+    BOLT11_HRP = SEGWIT_HRP
+    GENESIS = "000002fdc3921c1ad368816fcc587f499698d42b42ab5a5d94ee67882ef9d998"
+    DEFAULT_PORTS = {'t': '60001', 's': '60002'}
+    BIP44_COIN_TYPE = 1
+    LN_REALM_BYTE = 1
+
+    XPRV_HEADERS = {
+        'standard':    0x04358394,  # tprv
+        'p2wpkh-p2sh': 0x044a4e28,  # uprv
+        'p2wsh-p2sh':  0x024285b5,  # Uprv
+        'p2wpkh':      0x045f18bc,  # vprv
+        'p2wsh':       0x02575048,  # Vprv
+    }
+    XPRV_HEADERS_INV = inv_dict(XPRV_HEADERS)
+    XPUB_HEADERS = {
+        'standard':    0x043587cf,  # tpub
+        'p2wpkh-p2sh': 0x044a5262,  # upub
+        'p2wsh-p2sh':  0x024289ef,  # Upub
+        'p2wpkh':      0x045f1cf6,  # vpub
+        'p2wsh':       0x02575483,  # Vpub
+    }
+    XPUB_HEADERS_INV = inv_dict(XPUB_HEADERS)
+
+
 NETS_LIST = tuple(all_subclasses(AbstractNet))  # type: Sequence[Type[AbstractNet]]
 NETS_LIST = tuple(sorted(NETS_LIST, key=lambda x: x.NET_NAME))