Fix memory leak in call() retptr allocation

[ludfjig]

JavaScript components leak memory when returning non-primitive types (strings, objects, arrays, etc.).

Cause

Return pointer allocation used JS_realloc which bypasses memory tracking, so allocations were never freed.

Fix

Use cabi_realloc instead to ensure proper tracking and cleanup.

---

Before:

RES memory leaks indefinitely as can be seen in repro provided in #279:

Loading js component from: /home/ludde/leak/runner/../component/js/handler.wasm
Finished compiling component
Testing string echo (should leak)...
String Iteration: 0, Virtual: 5218636 KB, RES: 785404 KB
String Iteration: 500000, Virtual: 5218896 KB, RES: 838668 KB
String Iteration: 1000000, Virtual: 5218896 KB, RES: 845532 KB
String Iteration: 1500000, Virtual: 5218896 KB, RES: 852132 KB
String Iteration: 2000000, Virtual: 5218896 KB, RES: 855828 KB
String Iteration: 2500000, Virtual: 5218896 KB, RES: 862428 KB
String Iteration: 3000000, Virtual: 5218896 KB, RES: 870348 KB
String Iteration: 3500000, Virtual: 5218896 KB, RES: 878796 KB
String Iteration: 4000000, Virtual: 5218896 KB, RES: 886188 KB
String Iteration: 4500000, Virtual: 5218896 KB, RES: 893580 KB
String Iteration: 5000000, Virtual: 5218896 KB, RES: 902028 KB
.
.
.
String Iteration: 17000000, Virtual: 5218896 KB, RES: 1089204 KB
...

After:

RES memory stabilizes after X guest call iterations:

Loading js component from: /home/ludde/leak/runner/../component/js/handler.wasm
Finished compiling component
Testing string echo (should leak)...
String Iteration: 0, Virtual: 5349704 KB, RES: 779232 KB
String Iteration: 500000, Virtual: 5349964 KB, RES: 821192 KB
String Iteration: 1000000, Virtual: 5349964 KB, RES: 821192 KB
String Iteration: 1500000, Virtual: 5349964 KB, RES: 821192 KB
String Iteration: 2000000, Virtual: 5349964 KB, RES: 821192 KB
String Iteration: 2500000, Virtual: 5349964 KB, RES: 821192 KB
String Iteration: 3000000, Virtual: 5349964 KB, RES: 821192 KB
String Iteration: 3500000, Virtual: 5349964 KB, RES: 821192 KB
String Iteration: 4000000, Virtual: 5349964 KB, RES: 821192 KB
String Iteration: 4500000, Virtual: 5349964 KB, RES: 821192 KB
String Iteration: 5000000, Virtual: 5349964 KB, RES: 821192 KB
...

Closes #279

[vados-cosmonic]

Thanks for this @ludfjig ! @tschneidereit would you mind taking a look at this?

[tschneidereit]

Yeah, that's definitely the right thing to do—thank you for tracking this down!