Complete the deterministic loop core

[dhruvl]

Rounds out the loop core so real asyncio code runs on it with a full contract:

• unsupported asyncio APIs now raise a named SimulationFenceError instead of a bare NotImplementedError
• cancelled callbacks and timers are recorded in the scheduling trace, so replay hashes account for every RNG draw
• set_exception_handler / get_exception_handler and task factories, matching stdlib semantics on 3.12 and 3.13
• seeded sim.random / sim.uuid4 / sim.time shims: seed-derived inside a run, stdlib fallback outside
• replay stability proven across 100 reruns per seed and across processes under varying PYTHONHASHSEED
• scheduling overhead benchmark (stdlib only, no timing assertions)
• first docs page: the supported asyncio subset, stated honestly

45 tests green on 3.12 and 3.13, zero warnings, mypy strict clean.