WebCore: pass odd-offset UTF-16LE Blob slice child regression

Tracks a focused remaining item from #66.

The unchanged packages/runtime/test/test/js/web/fetch/blob.test.ts case Blob.slice at an odd byte offset decodes UTF-16LE (BOM) content with text() and json() still fails in Home. In ReleaseFast the spawned child returns empty stdout rather than the expected JSON payload; the pinned Bun control passes.

Diagnose and port the real subprocess/Blob decoding behavior. Do not lengthen the fixture timeout, replace the child process, skip the test, or hard-code its payload.

Acceptance: the unchanged test passes in Debug and ReleaseFast with the original workload and deadline.

Fixed by d25fc0bd. UTF-16LE Blob text/json decoding now keeps the aligned fast path and decodes odd-address slices into aligned temporary u16 storage, dropping only an incomplete trailing byte. The unchanged subprocess regression passes with its original deadline in Debug and ReleaseFast; all 16 Blob/Body OOM cases also remain green.