chore: drop unused imports and a dead constant (no behaviour change)

- backtest/diagnose_filters: unused BacktestConfig import
- pair_signals/__init__: unused PlatformConfig import
- scripts/shadow_scan: unused pandas import
- research burst_precursors / calendar_impact / duka_spread: unused sys / COST / struct
- test_all: MagicMock, StringIO, Colors and two function-local imports that
  nothing in the test bodies used
- telegram_bot: ViolentWatch is only a type hint -> imported under TYPE_CHECKING

compare_oanda_tiingo.py and oanda_td_divergence.py are left alone here because
the hygiene commit deletes them.

Suite: 464 passed, 0 failed (unchanged).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0156YhogLytzj9P2iH1skGbL

test_all.py

@@ -12,8 +12,7 @@ import threading
 import tempfile
 import traceback
 from datetime import datetime, timedelta, timezone
-from unittest.mock import patch, MagicMock
-from io import StringIO
+from unittest.mock import patch
 
 import numpy as np
 import pandas as pd
@@ -1489,7 +1488,7 @@ print("\n═══ Module 6: main.py ═══")
 
 from main import (
     signal_color, quality_color, _compact_reason,
-    report_to_dict, build_config, Colors,
+    report_to_dict, build_config,
 )
 
 
@@ -2777,7 +2776,6 @@ def _():
 
 @test("CP: cp_patterns subset filter excludes disabled detectors")
 def _():
-    from dataclasses import replace
     df = make_cp_df(double_bottom_closes())
     cfg = XauStrategyConfig(cp_patterns=["bull_flag"])
     assert detect_chart_pattern(df, atr=5.0, cfg=cfg) is None
@@ -7757,7 +7755,6 @@ def _():
 @test("signal_log: analyze_pair weekend early-return is logged")
 def _():
     import signal_log as slog
-    from signal_aggregator import _is_forex_weekend
     fd, path = tempfile.mkstemp(suffix=".db")
     os.close(fd)
     os.remove(path)