Add wallet key derivation and PSBT building

BIP84 derivation of per-user P2WPKH addresses and the pool address from
the encrypted master xprv (app/wallet/hd.py, keystore.py), the PLM
mainnet chain params (plm_network.py), and PSBT construction for bets/
payouts/withdrawals with change-output and fee-estimation logic
(psbt_builder.py).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/unit/test_hd.py

@@ -0,0 +1,21 @@
+from embit.bip32 import HDKey
+
+from app.wallet.plm_network import PLM_MAINNET
+
+
+def test_p2wpkh_address_uses_plm_hrp():
+    from embit import script
+
+    root = HDKey.from_seed(b"\x01" * 32, version=PLM_MAINNET["xprv"])
+    child = root.derive("m/84h/746h/0h/0/0")
+    address = script.p2wpkh(child.to_public()).address(network=PLM_MAINNET)
+    assert address.startswith("plm1q")
+
+
+def test_derivation_is_deterministic():
+    root = HDKey.from_seed(b"\x02" * 32, version=PLM_MAINNET["xprv"])
+    a = root.derive("m/84h/746h/0h/0/5").sec()
+    b = root.derive("m/84h/746h/0h/0/5").sec()
+    c = root.derive("m/84h/746h/0h/0/6").sec()
+    assert a == b
+    assert a != c