[bbdu3]

Summary

NetworkTransport previously skipped an entire receive chunk when it began with a heartbeat, discarding any following JSON-RPC messages. Parse buffered frames so the heartbeat is consumed without losing subsequent messages.

Fixes #322.

Changed

• Move heartbeat detection into the buffered receive loop.
• Consume only the complete 12-byte heartbeat, then continue processing remaining bytes.
• Retain incomplete frames for the next read.
• Add 14 parameterized regression cases covering coalesced heartbeats and messages, plus splits across the heartbeat and following JSON.
• Preserve public APIs and the existing wire format.

Test

• Python behavior-model checks: all 14 regression inputs fail under the original logic and pass under the revised logic. Additional fragmentation and control cases passed.
• git diff --check passed.