feat: add configurable routing fee leeway for Loop Out off-chain payments

[TrezorHannes]

Summary of Changes

• Root Cause Resolution: Addressed rigid fee limit capping in loop out that previously passed --max_swap_routing_fee with only a tight 1.5x buffer, causing LND/loopd pathfinding to fail with FAILURE_REASON_NO_ROUTE when attempting multi-path (MPP) shards or alternative intermediate hops.
• Configurable Routing Fee Leeway:
  • Implemented calculate_max_routing_fee_budget() with default 100% leeway (2.0x headroom) and 500 sat base buffer.
  • Added --fee-leeway-pct CLI flag and fee_leeway_pct / fee_leeway_base_sats options in config.ini.
  • Allowed passing --max-routing-fee 0 to explicitly omit fee limits and let Loop daemon use its native default.
• Enhanced Operator UX:
  • Displays both estimated probed fee and maximum routing fee budget (with satoshi leeway) on CLI before dispatching.
  • Updated simulated command in --dry-run to accurately print fee budget parameters.
• Testing & Documentation:
  • Added 5 new unit tests in tests/Other/test_swap_out_loop.py (96/96 tests pass).
  • Updated Other/swap_out-loop.md and config.ini.example.