docs: Add detailed iperf3_lwip implementation and debugging plan.

Comprehensive plan covering:
- Code review identifying potential boot failure causes
- Bisection strategy to isolate boot issue (Phases 0-6)
- Full implementation roadmap (TCP/UDP/Python integration)
- Performance targets and testing strategy
- Commit strategy for iterative development

Primary hypothesis: Static state structure initialization causing boot
failure when combined with other user C modules.

Plan includes fallback to heap-allocated state if static init fails.

Signed-off-by: Andrew Leech <[email protected]>

Author: pi-anl