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>

trip_planner.py

@@ -5,6 +5,7 @@ Combine GWR station→Paddington trains with Eurostar St Pancras→destination t
 from datetime import datetime, timedelta
 
 import circle_line
+from tfl_fare import circle_line_fare
 
 MIN_CONNECTION_MINUTES = 50
 MAX_CONNECTION_MINUTES = 110
@@ -31,7 +32,11 @@ def _circle_line_services(arrive_paddington: datetime) -> list[dict]:
     )
     services = circle_line.upcoming_services(earliest_board, count=2)
     return [
-        {"depart": dep.strftime(TIME_FMT), "arrive_kx": arr.strftime(TIME_FMT)}
+        {
+            "depart": dep.strftime(TIME_FMT),
+            "arrive_kx": arr.strftime(TIME_FMT),
+            "fare": circle_line_fare(dep),
+        }
         for dep, arr in services
     ]