feat(transport): Multipath-QUIC preparation

[Yanstart]

Context

IETF Multipath-QUIC is coming. The draft enables using multiple network paths simultaneously — this is transformative for satellite: bonding LEO + GEO for reliability, or satellite + terrestrial for seamless handovers with bandwidth aggregation.

Use cases:

• LEO + GEO bonding: LEO for low latency, GEO for reliability during LEO handovers
• Satellite + terrestrial: seamless failover when entering/leaving satellite coverage
• Dual satellite: two LEO operators for redundancy (Starlink + OneWeb)

Standards alignment: 3GPP 5G-Advanced (Rel-18+) defines multi-connectivity for NTN. Multipath-QUIC is the transport-layer mechanism to implement this. Our architecture must be ready.

Relationship with #42: Dynamic parameter switching handles single-path adaptation. Multipath handles multiple simultaneous paths — they're complementary, not competing.

Requirements

• Architecture must support multiple QUIC connections per tunnel session
• Connection manager abstraction (single path → multi path)
• Path quality monitoring per path (RTT, loss, bandwidth, jitter)
• Scheduler interface (pluggable strategy):
  • Redundant: same data on all paths (reliability)
  • Round-robin: alternate packets across paths (bandwidth aggregation)
  • Weighted: proportional to path quality (smart aggregation)
  • Lowest-RTT: prefer fastest path (latency optimization)

Implementation

1. Refactor connection management to support path abstraction
2. Add path quality metrics (RTT, loss, bandwidth per path)
3. Design scheduler trait (pluggable scheduling strategy)
4. Monitor IETF MP-QUIC draft progress
5. Do NOT implement draft — prepare architecture only
6. Coordinate with feat(transport): Dynamic satellite/terrestrial parameter switching #42 (path quality feeds into dynamic profiles)

Dependencies

• feat(transport): Dynamic satellite/terrestrial parameter switching #42 (dynamic parameters per path)
• feat(transport): BBRv3 readiness and abstraction layer #48 (CC must support per-path state for multipath)

Acceptance Criteria

• Connection manager supports path abstraction
• Single-path mode unchanged (no regression)
• Architecture documented for MP-QUIC integration
• Scheduler trait defined with at least 2 strategies stubbed
• Path quality monitoring infrastructure ready