Make the walkthrough seed opt-in: unseeded FrotzEnv is now stochastic (v4.0)

[Copilot]

FrotzEnv(rom) without a seed silently applied the walkthrough seed (e.g. 12 for Zork I) for any game with bindings, i.e. the whole benchmark suite. Callers who reasonably assumed "no seed → stochastic" were unknowingly running deterministic episodes, applying the fixed-seed handicap the Jericho paper asks researchers to disclose. reset()'s docstring already advertised a use_walkthrough_seed parameter that was never implemented.

This is a breaking change; version bumped to 4.0.0.

API changes (jericho/jericho.py)

• seed(None) now resolves to -1 (time-dependent) instead of bindings['seed']. As a side effect, an explicit seed=0 is now honored — the previous seed or bindings.get(...) treated it as unset.
• reset(use_walkthrough_seed=False) implements the documented toggle; True seeds the emulator with the walkthrough seed for that episode, and warns (UnsupportedGameWarning) if the game has none.
• New FrotzEnv.walkthrough_seed property exposing the game's walkthrough seed, or None.
• New ImplicitRandomSeedWarning raised on reset() when the game has a walkthrough seed and the caller supplied neither an explicit seed nor use_walkthrough_seed — silent for anyone who made the choice explicit.

Callers and docs

• tools/test_games.py, tools/find_walkthrough.py and tests/test_jericho.py::test_copy replay walkthroughs, so they now reset with use_walkthrough_seed=True.
• README gained a "Breaking changes in Jericho 4.0" section; the quickstart walkthrough section documents the new reset argument and flags seeding as a handicap.
• tests/test_seed.py covers seed resolution, the new property, warning behavior, and walkthrough reproducibility.

env = FrotzEnv("zork1.z5")            # stochastic (time-dependent seed)
env = FrotzEnv("zork1.z5", seed=-1)   # stochastic, explicit → no warning
env.walkthrough_seed                  # 12
env.reset()                           # warns: walkthrough seed no longer implicit
env.reset(use_walkthrough_seed=True)  # deterministic, reproduces get_walkthrough()

• Fixes Silent fixed-seed default prevents independent runs for most games. Consider adding a warning? #84

[abahgat]

I reviewed this and folded it into #88, keeping the commit intact since it was a good start. The downstream PR includes a few fixes and stronger tests. I am happy to continue on #88 if that looks good to you (there are a couple open questions for you)

[MarcCote]

Superseded by #88