Add StyleThreading::Sequential opt-out for concurrent document resolves

#437 in DioxusLabs/blitz — merged 2026-05-12

Adds a StyleThreading::Sequential opt-in on DocumentConfig for callers that need to resolve documents from multiple threads concurrently (#430); the gate sits at the traverse_dom call site in packages/blitz-dom/src/stylo.rs rather than via layout.threads, because the pref is read once when STYLE_THREAD_POOL is first forced and would silently no-op for any document constructed after the pool exists.

Default stays Parallel; verified locally against the issue repro across all-Sequential, all-Parallel (panics, as a negative control), and pool-already-initialized mixed-mode scenarios.