[Task] Design virtual thread support for ShenYu runtime and plugins

### Description

## Background

A future ShenYu main repo change may use Java virtual threads to improve blocking IO handling in runtime and plugin execution paths.

Before implementation, we need a design that identifies where virtual threads are useful, where they are risky, and how to keep a fallback path.

## Tasks

- [ ] Inventory current thread pool usage in bootstrap/runtime/plugin code.
- [ ] Identify blocking IO paths that may benefit from virtual threads.
- [ ] Identify CPU-bound paths that should not use virtual threads.
- [ ] Design a unified executor abstraction for plugin/runtime use.
- [ ] Define configuration flags for enabling/disabling virtual threads.
- [ ] Define fallback behavior when virtual threads are unavailable.
- [ ] Define observability requirements: thread naming, metrics, tracing, logs.
- [ ] Define benchmark scenarios comparing platform threads and virtual threads.
- [ ] Identify compatibility impact on plugins.

## Expected Output

A virtual thread design proposal covering:

- Candidate execution paths
- Non-candidate execution paths
- Executor abstraction
- Configuration model
- Fallback model
- Benchmark plan
- Risk analysis

## Acceptance Criteria

- The proposal does not require Java client code to upgrade JDK.
- The proposal includes a safe fallback path.
- The proposal includes measurable benchmark criteria.

### Task List

_No response_

Start by inventorying thread-pool usage in the repository's bootstrap, runtime, and plugin code, then trace blocking I/O and CPU-bound execution paths. The work is done when the proposal covers candidate and non-candidate paths, executor and configuration models, safe fallback behavior, observability, benchmarks, compatibility, and the stated acceptance criteria.

Written by the indexing model from the issue text.