stp: integrate 802.1D Spanning Tree Protocol into BridgeDevice
Adds loop prevention to Ethernet bridges:
- BridgeDevice gains stp: Option<StpState> field
- enable_stp(priority, mac) method initializes STP per-bridge
- BPDU frames (dst 01:80:c2:00:00:00) intercepted in recv(),
processed locally, never forwarded
- STP hello timer sends periodic BPDUs on all ports (root bridge)
- flood() skips STP-blocked ports
- build_bpdu() made public for bridge integration
- stp module declared in link/mod.rs
The recv() flow now: age MACs → check STP hello timer →
poll ports → detect BPDU (absorb) → normal frame (learn + forward).
Reference: Linux 7.1 net/bridge/br_stp.c, br_stp_bpdu.c, br_stp_timer.c
Red Bear OS
30db94c970