Survive oversized Electrum replies and ignore dust deposits (B-75, ELC-20, DEP-01, DAT-10)

A reply line over asyncio's default 64 KiB StreamReader limit raised out of
_read_loop and closed the only connection; the pool address's get_history passes
that after ~800 transactions, and dust sent to any public address does it on
purpose.

- ElectrumClient opens the stream with a 16 MiB limit and frames lines itself:
  an oversized line is discarded up to its newline, a non-JSON line is dropped,
  and the request whose id is readable in the first/last 256 bytes fails at once;
  the connection stays up either way.
- Coins below DUST_LIMIT_SATS are never offered as credit candidates, never
  credited, and select_utxos never selects them.
- Spec (ELC-20, DEP-01, DAT-10, ARC), storico, BUGS.md and test counts updated.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/unit/test_psbt_builder.py

@@ -32,6 +32,16 @@ def test_select_utxos_picks_largest_first():
     assert total == 20_000_000
 
 
+def test_select_utxos_ignores_dust():  # B-75 (DEP-01)
+    from app.wallet.psbt_builder import DUST_LIMIT_SATS
+
+    utxos = [Utxo("a" * 64, 0, DUST_LIMIT_SATS - 1), Utxo("b" * 64, 0, DUST_LIMIT_SATS)]
+    selected, total = select_utxos(utxos, target_sats=DUST_LIMIT_SATS)
+    assert [u.txid for u in selected] == ["b" * 64]
+    with pytest.raises(InsufficientFundsError):
+        select_utxos(utxos, target_sats=DUST_LIMIT_SATS + 1)
+
+
 def test_select_utxos_raises_when_insufficient():
     utxos = [Utxo("a" * 64, 0, 1_000_000)]
     with pytest.raises(InsufficientFundsError):