WebCore: preserve Blob object-part stringification order

Tracks a focused remaining item from #66.

The unchanged packages/runtime/test/test/js/web/fetch/blob.test.ts case new Blob stringifies non-Blob object parts in order still fails in Home. A URL part alone stringifies correctly, but the mixed parts ["a", url, "b"] produce duplicated adjacent content instead of ahttps://example.com/pathb. The pinned Bun control passes.

Port the real upstream Blob part coercion/join behavior and its ownership rules. Do not rewrite the fixture, special-case URL, or weaken the assertion.

Acceptance: the unchanged test passes in Debug and ReleaseFast and neighboring Blob lifetime tests remain green.

Fixed by d25fc0bd. Blob multipart construction now visits parts in source order, stringifies non-Blob wrappers and nested arrays exactly once in place, and retains immediate byte snapshots for buffer/Blob parts. The unchanged main Blob fixture passes 26/26 in Debug and ReleaseFast; blob-array-fast-path passes 11/11, including 10,000 parts, sparse arrays, prototype indexed getters, and mixed parts.