Add Circle line fare to Transfer column and Total

Add tfl_fare.py with circle_line_fare() which returns £3.10 (peak) or
£3.00 (off-peak) based on TfL Zone 1 pricing. Peak applies Monday–Friday
(excluding England public holidays) 06:30–09:30 and 16:00–19:00.

Annotate each circle service with its fare in trip_planner.py, display
it alongside the Circle line times in the Transfer column, and include
it in the journey Total.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tfl_fare.py

@@ -0,0 +1,30 @@
+"""TfL single fare calculations for journeys within Zone 1."""
+
+from datetime import datetime, time
+
+import holidays
+
+CIRCLE_LINE_PEAK = 3.10
+CIRCLE_LINE_OFF_PEAK = 3.00
+
+_ENGLAND_HOLIDAYS = holidays.country_holidays("GB", subdiv="ENG")
+
+_AM_PEAK_START = time(6, 30)
+_AM_PEAK_END = time(9, 30)
+_PM_PEAK_START = time(16, 0)
+_PM_PEAK_END = time(19, 0)
+
+
+def circle_line_fare(depart_dt: datetime) -> float:
+    """Return the TfL Circle line single fare for a given departure datetime.
+
+    Peak (£3.10): Monday–Friday (excluding public holidays),
+                  06:30–09:30 and 16:00–19:00.
+    Off-peak (£3.00): all other times, weekends, and public holidays.
+    """
+    if depart_dt.date() in _ENGLAND_HOLIDAYS or depart_dt.weekday() >= 5:
+        return CIRCLE_LINE_OFF_PEAK
+    t = depart_dt.time()
+    if _AM_PEAK_START <= t < _AM_PEAK_END or _PM_PEAK_START <= t < _PM_PEAK_END:
+        return CIRCLE_LINE_PEAK
+    return CIRCLE_LINE_OFF_PEAK