perf: use calculated CWND for terrestrial profile

[Yanstart]

Context

Terrestrial profile hardcodes initial_cwnd_packets=32 (43KB). BDP at 100Mbps/10ms = 125KB. BBR2 needs 3-4 RTTs to probe up, interacting badly with the pacing bottleneck.

All other profiles use calculate_optimal_cwnd() but terrestrial has a manual override.

Fix

Change terrestrial initial_cwnd_packets from 32 to 100 (135KB, covers 10ms BDP at 100 Mbps).

File

• crates/quick-transport/src/config.rs line 393

Acceptance Criteria

• Terrestrial initial_cwnd_packets >= 100
• BBR2 reaches line rate within 2 RTTs (20ms) on terrestrial
• 351+ tests pass