docs(vmcp): document core transport split

[buyicoder]

Summary

• Document the current vMCP core/transport split in the architecture overview and embedding guide.
• Refresh the embedding guidance and stability table for pkg/vmcp/core, server.Serve, and the stable server.New wrapper.
• Update the root pkg/vmcp package docs to describe core.VMCP, subtract-only decorators, and the supported outer handler wrapping pattern.

Test plan

• git diff --check
• Not run: gofmt, go test ./pkg/vmcp/..., and task docs because gofmt, go, and task are not available in my local PATH.

Closes #5446.

[buyicoder]

Thanks for the detailed issue write-up. I kept this docs-only and aligned the wording with the current pkg/vmcp/core + pkg/vmcp/server split I found in main, including the stable server.New wrapper path.

I could only run git diff --check locally because Go/Task are not installed in this environment, so I called that out in the PR body.