Module backpressure 

Credit-Based Backpressure & Flow Control — FR-003

Implements credit-based flow control for K2K messaging:

• Producer requests credits from consumer before sending
• Consumer grants credits based on available queue capacity
• When credits exhausted: producer blocks, buffers, or drops (configurable)
• Queue watermark signals: LOW_WATER / HIGH_WATER
• Per-channel flow control

Protocol

Producer                          Consumer
   │                                  │
   │── RequestCredits(n) ──────────►  │
   │                                  │── GrantCredits(n) ─►
   │◄── GrantCredits(n) ──────────── │
   │                                  │
   │── Message (credit-1) ─────────► │
   │── Message (credit-1) ─────────► │
   │── Message (credit-1) ─────────► │
   │   (credits exhausted)            │
   │── RequestCredits(n) ──────────►  │
   │   ...                            │

Structs

FlowControlConfig

Flow control configuration for a K2K channel.

FlowController

Per-channel flow controller.

FlowMetrics

Flow control metrics.

FlowMetricsSnapshot

Snapshot of flow control metrics (non-atomic, for reporting).

Enums

OverflowPolicy

Policy when producer has no credits left.

WatermarkSignal

Watermark signal emitted when queue depth crosses thresholds.