`StateProxy` looked like a thread-safety boundary, but it only protected
a small slice of state operations. Some examples of operations that were
not covered:
- `self.state.counter += 1`, `self.state["counter"] += 1` (increments)
- `self.state.user.profile.score += 1` (nested object mutations)
- `self.state.config["limits"]["max"] = 10` (mutation through model fields)
- `self.state.items[0].status = "done"` (list/container mutations)
This commit decided to remove it completely for simplicity and
performance:
- Simpler runtime code
- attr read: 24x faster, attr write: 27x faster, list append: 19x faster (local benchmark)
- Clearer concurrency contract (lifecycle locks remain, but arbitrary
shared state mutation is not presented as thread-safe)
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