The `jev-ultrafast` library API's `Agent` initialization and observation mechanism, specifically the handling of initial DOM snapshots and agent termination logic.

# Library API: first observation can return an empty action space; agent terminates with BLOCKED instead of retrying

## Summary

When the library API creates `Agent(url)`, the first DOM observation can return a snapshot with **zero indexed elements** while URL and title are already correct. The policy receives this near-empty action space and terminates the run by choosing `BLOCKED` (~0.8 confidence). The run ends with status `blocked` in under a second, with no retry.

## Evidence

`Browser.__init__` waits for `document.readyState == "complete"`, but the loop can sample `readyState` from the pre-navigation `about:blank` document (instantly `"complete"`) before `Page.navigate` commits. Immediately after init, the real document is still at `interactive`:

```python
from jev_ultrafast.browser import Browser
b = Browser("saucedemo.com/inventory.html&qu...
print(b.evaluate("document.readyState")) # -> 'interactive' (the wait loop already exited)
```

`observe()` retries only on `StalePage` exceptions; a successful but degenerate snapshot is returned as-is. Consecutive observations right after init on the same page:

```text
observe #1: url=.../inventory.html title='Swag Labs' elements=0
observe #2: url=.../inventory.html title='Swag Labs' elements=0
observe #3 (≈1s later): elements=19
```

Repro of the terminal effect, 6/6 cold starts (repro script below):

```text
trial 1: first-observe elements=0 -> model chose BLOCKED (0.79)
trial 2: first-observe elements=0 -> ...