refactor: enforce frozen natively in StrictBaseModel

[tcoratger]

cc @alexanderlhicks

What

Enforce immutability once, in the base model, instead of per class:

• StrictBaseModel now carries "frozen": True natively (restoring the pre-refactor: drop model_copy in favour of in-place mutation #789 constraint and docstring).
• The 17 per-class model_config = Container.model_config | {"frozen": True} overrides from refactor(lstar): freeze constructed-once SSZ containers #842/refactor(lstar): freeze the Block family #843 are deleted — subclasses inherit the constraint.
• No mutable opt-outs. State and Store are frozen too, and every remaining in-place mutation site is converted back to the model_copy(update=...) functional style.

Follow-up to #842/#843, as discussed in the #843 review thread.

Why

Immutability is groundwork for formal verification: a block, checkpoint, state, or store never changes once it exists, so instances are safe to share by reference across fork choice, the chain, and attestations. Enforcing it in the base keeps the spec legible — the constraint lives in one place rather than being repeated on every container.

Migrated mutation sites

Spec (src/lean_spec/spec/):

• state_transition.py — process_slots rebinds through model_copy (the deepcopy barrier is no longer needed), process_block_header applies its updates atomically in one final copy, process_attestations returns a new state.
• fork_choice.py, timeline.py, validator_duties.py, aggregation.py — every store update flows through model_copy; dicts and inner sets are shallow-copied before growing so the caller's store is left untouched.
• xmss/interface.py — advance_preparation returns a rebuilt secret key instead of rotating the bottom trees in place.

Node (src/lean_spec/node/):

• chain/service.py, sync/service.py — rebind the store instead of patching it in place.
• networking/enr/enr.py — from_rlp rebuilds the record with the computed node id.

Test framework + tests:

• packages/testing fixtures and ~110 test-side fixture-setup mutations converted to model_copy rebinding; helpers that mutated arguments now return the new instance.
• Restores test_frozen_rejects_assignment (dropped in refactor: drop model_copy in favour of in-place mutation #789) and the immutability wording in the SSZ patterns rule; adds mirrored immutability tests for State and the new containers/test_store.py.

Caveat

Unchanged from #842/#843: frozen is an enforcement aid, not a hard immutability guarantee (pydantic#12361).

Validation

• just check — clean (ruff lint + format, ty, codespell, mdformat, lock)
• Unit suites: lstar spec 257 passed, node 1682 passed, crypto/ssz/enr/containers/base 1660 passed
• Every migration preserves hash_tree_root, so generated fixtures should be byte-identical; CI's fill run will confirm.

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