Port zero-dependency stochastic core

[nilp0inter]

Summary

• Re-scope Montest from BDD-only positioning to a zero-runtime-dependency stochastic testing core.
• Add typed core primitives, sync/async sequential iterators, SPRT, and explicit AllOf/AnyOf composites.
• Add core behavior, async, SPRT, composite, and deterministic integration tests.

Verification

• nix develop --command task sync
• nix develop --command task lint
• nix develop --command task typecheck
• nix develop --command task test
• nix develop --command uv run python -c 'import math, random; from montest import Decision, SequentialIterator, sprt; llr=lambda x: math.log(0.6/0.3) if x else math.log(0.4/0.7); rng=random.Random(42); samples=list(SequentialIterator(lambda: int(rng.random() < 0.6), sprt(llr=llr, alpha=0.05, beta=0.10))); assert samples[-1].decision is Decision.ACCEPT_H1; print(len(samples), samples[-1].decision.value)'
• nix develop --command uv build
• Wheel metadata inspected: no gherkin-official runtime dependency; no pytest/behave extras; dev extra remains; py.typed included.

Note: pytest and behave integration extras are intentionally deferred; this change ports the zero-dependency stochastic core only.