[WIP] Add standalone control-aware bounded channel for future engine integration

[lquerel]

** Not ready for review yet **

Change Summary

Add a new standalone control-channel crate that defines a bounded control-aware channel for OTAP node-control traffic.

This change is intended as a first step toward the broader engine redesign tracked in #2465. It introduces the control-channel design, tests, benchmarks, and documentation in a standalone form so the control-plane semantics can be reviewed and validated before integration into the engine runtime.

The new channel is designed for the control-path requirements of the OTAP engine:

• reserved lifecycle delivery for DrainIngress and Shutdown
• bounded and batched Ack / Nack handling
• coalesced best-effort control for TimerTick / CollectTelemetry
• latest-wins Config
• bounded fairness and deadline-bounded terminal shutdown progress
• role-specific APIs for receiver vs non-receiver nodes
• a single-owner implementation aligned with the engine's thread-per-core model
• FIFO blocked-sender wakeups for completion-capacity backpressure
• explicit completion metadata support via AckMsg<PData, Meta> /
  NackMsg<PData, Meta> for future engine unwind-state integration

This PR does not yet wire the new control channel into receivers, processors, exporters, or pipeline_ctrl. That integration is planned in the later steps of #2465.

What issue does this PR close?

• Part of Improve control delivery, shutdown robustness, and local deferred work #2465
• Prepares Guarantee deadline-forced shutdown delivery under saturated node-control channels #2431

How are these changes tested?

• Added unit tests for the standalone control-channel crate covering:
  • lifecycle acceptance and ordering
  • completion batching
  • bounded fairness
  • shutdown deadline behavior
  • blocked-sender wake behavior and cancellation safety
  • completion metadata preservation
  • invalid config rejection
• Added a benchmark comparing the standalone control-aware channel against the
  current engine control-channel paths under heavy Ack / Nack traffic and
  mixed control noise
• Verified with: cargo xtask check

Benchmark results

Ack/Nack Only

Mode	Implementation	Time (ms)	Throughput (Melem/s)	Delta vs current
Local	current_local	7.22	13.85	baseline
Local	control_aware	6.39	15.64	~+12.9% thrpt / -11.4% time
Shared baseline	current_shared	16.68	5.99	reference only

Ack/Nack + Control Noise

Mode	Implementation	Time (ms)	Throughput (Melem/s)	Delta vs current
Local	current_local	7.51	13.31	baseline
Local	control_aware	6.49	15.42	~+15.8% thrpt / -13.6% time
Shared baseline	current_shared	17.01	5.88	reference only

Notes:

• control_aware is the standalone single-owner channel introduced in this PR.
• current_shared is included as a reference against the current engine shared path.
• The benchmark still runs on a current-thread runtime and is intended to compare the current engine paths with the standalone single-owner control-channel design before engine integration.

Are there any user-facing changes?

No user-facing behavior changes yet.

This PR adds the standalone control channel and its validation/benchmarking surface, but does not yet integrate it into the engine runtime.