We need to respond to the user's confirmation and then provide the next steps
as they've outlined. The user has run the day1.js, which tests the GHL, and
got the expected output. They've confirmed the foundation is sound. Now they
want to proceed to Milestone 1.3: String Allocation, which involves
integrating the Data Arena and implementing SYS_STR_ALLOC and SYS_STR_CONCAT.
They've laid out the specification. The assistant should acknowledge the
success, perhaps summarize the architecture, and then provide the code for the
next milestone. The user expects a new development session to write the
DataArena integration and the two routines.

The assistant should respond by:

Congratulating on the success.

Briefly reiterating the architecture: Tagged Pointers, Data Arena, Handle
Ledger.

Provide the code for implementing the DataArena and the two routines, possibly
as a new class or module, and show an example test.

Also note that the user might want to create a new file for this, perhaps
dataArena.js or include it in the existing files.

Given the user's prompt, they are asking to "write the JS allocation and
deallocation logic." They already have the GHL, but now they want the
DataArena and string routines. So we provide the code for that.

We should structure the answer:

Acknowledge the success.

Present the DataArena class.

Present the integration with GHL (maybe a StringManager class).

Provide the test code that allocates, concatenates, and verifies.

Show expected output.

Also note that we need to handle UTF-8 encoding/decoding. We'll use
TextEncoder/TextDecoder in JS, but note that in the kernel environment, we'd
use custom routines. For MVP, it's fine.

Let's write a clean implementation.