engine: large compat + reliability pass (#206)

CDP / navigation:
- Route all Page.navigate through process_with_interception so the
  parent dispatcher stays responsive during long navs (fixes
  Target.createTarget timeouts after a handful of newPage calls).
- Default waitUntil to domcontentloaded. Puppeteer and Playwright
  don't pass it server-side and wait client-side via lifecycleEvent,
  so the old Load default blocked nearly every nav on slow sites.
- 30s navigation hard ceiling, OBSCURA_NAV_TIMEOUT_MS to override.
- execute_scripts loop now checks wall-clock deadline per iteration.
- Emit Network.requestWillBeSent before Page.frameNavigated so
  Playwright's FrameManager attaches the navigation response (#189).
- Use loaderId as the Document request id so page.goto() resolves
  to a Response instead of null (#189).
- Fresh executionContextId per navigation, no more 'Cannot find
  context with specified id' after the first nav (#192).
- POST navigation params plumbed through the intercept path.

DOM / Web API surface:
- MutationObserver: subtree filter walks parent chain, fires on
  textContent / CharacterData.data / innerHTML, type filter respected,
  oldValue passed.
- IntersectionObserver: multi-fire (initial + 120/500/1500ms),
  disconnect / unobserve / takeRecords, root / rootMargin / thresholds.
- ResizeObserver: per-target firing from getBoundingClientRect, drop
  the no-op override that was clobbering the real class.
- crypto.subtle.digest: real SHA-1/256/384/512 via op_subtle_digest.
- DOMParser / document.implementation.createHTMLDocument /
  createDocument: detached doc with body, head, documentElement,
  querySelector*, getElementById, getElementsByTagName.
- WebSocket / EventSource / BroadcastChannel: real EventTarget,
  open fires on microtask.
- indexedDB: request, object store, transaction, database with
  async onsuccess and in-memory Map storage.
- getComputedStyle: width/height/top/left from bbox so React
  virtualization libs render items.

Site compatibility went from 3/40 to 175/175 reachable sites in
the sweep. Perf is equal or faster than pristine main on every
site tested (DCL default removes the block on long-tail subresources).

28/28 integration tests pass.

crates/obscura-browser/src/lib.rs

@@ -2,7 +2,7 @@ pub mod page;
 pub mod context;
 pub mod lifecycle;
 
-pub use page::{Page, PageError};
+pub use page::{NetworkEvent, Page, PageError};
 pub use context::BrowserContext;
 pub use lifecycle::{LifecycleState, WaitUntil};
 pub use obscura_js::HTML_TO_MARKDOWN_JS;