refactor(testing): replace the pre-state auto-inject with field defaults

[tcoratger]

Motivation

Item 6 of the packages/testing/ audit (follow-up to #848–#851). The pre-state default was a hybrid of two competing mechanisms:

• FixtureWrapper.__init__ sniffed each fixture class's __annotations__ for a pre/anchor_state field and silently injected a pytest-fixture-provided genesis state when the test omitted one
• yet 47/53 state_transition_test calls passed pre=generate_pre_state() explicitly anyway, while 47/84 fork_choice_test calls relied on the magic

Worst of both worlds: the default was invisible (the model declared pre: State required and a hidden wrapper papered over it), and the test suite had no consistent style.

Changes

The magic dies; the default moves onto the model field — visible, standard Pydantic:

pre: State = Field(default_factory=generate_pre_state)            # StateTransitionTest
anchor_state: State = Field(default_factory=generate_pre_state)   # ForkChoiceTest, VerifySignaturesTest

• The pre pytest fixture, the annotation sniffing, and the wrapper kwargs injection are deleted (the request.param indirect branch was verified unused)
• The now-always-set fields lose their | None types, None-branches, and assert ... is not None guards
• Lstar.spec_class() lost its only caller (the deleted pre fixture) and is removed
• One test convention (~34 lines stripped): the standard genesis pre-state is omitted; only non-default pre-states are spelled out (generate_pre_state(num_validators=8), advanced states, build_anchor(...)) — deviations from the baseline are now visually loud

Latent import-cycle fix in the spec

Removing the accidental LstarSpec import from the consensus forks module exposed a real bug: import lean_spec.spec.crypto.xmss crashes on current main with a circular-import error (xmss/containers.py → forks.lstar.slot → forks/__init__ → aggregation.py → back into the partially-initialized xmss containers). It only ever worked because some other module happened to load the forks package first. Fix at the root: xmss/__init__.py now loads the forks package before its own containers, making the cycle resolve from any entry point (verified: xmss-first, forks-first, and consensus_testing imports all succeed).

Vector impact: none

The default factory produces exactly the state the deleted pytest fixture produced (same 4-validator genesis). The pre-state is always serialized into the emitted JSON, so clients see no difference.

Verification

• just check passes (ruff, format, ty, codespell, mdformat)
• Import smoke from all three entry orders passes
• Smoke fill across all three affected fixture types (state_transition, verify_signatures, fork_choice — default and explicit paths): 14/14 pass

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