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bobzhang 
test(lazy_list): add extensional and intensional QuickCheck specs

A lazy list has two specifications that must hold at once, and a bug in
either is invisible to a test that only checks the other.

The extensional half — what elements come out — is settled by an Array
model: every combinator is mirrored by a three-line strict definition,
and the two must agree. The properties drive whole randomly generated
*pipelines* rather than single combinators, because the interesting
failures in a lazy structure live in the composition (flat_map feeding
take, concat feeding zip) and not in any one operation. Alongside that
sit the algebraic laws: functor identity and composition, flat_map's
singleton and annihilator cases, concat as a monoid, take/drop as a
partition, take_while/drop_while splitting at the same point, filter
idempotence and commutativity, and zip's two projections.

The intensional half — how much is forced, and when — is what makes
the structure worth having, and the doc comments state it precisely:
take forces "exactly the cells of the prefix -- no look-ahead", drop
forces "up to n cells", head "does not force the tail", tails memoize
so a thunk runs "at most once", and concat does not touch its
right-hand side until the left is exhausted. Those are pinned here as
*exact thunk counts* against an instrumented infinite source. An
implementation that quietly forced one cell too many would still pass
every extensional test, but would turn take on an infinite list from a
total function into a hang -- so the counts are asserted as equalities,
not bounds, and a final property re-runs every lazy combinator against
an infinite source to confirm it stays total.

All 20 tests pass on wasm, wasm-gc, js and native.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

16012 of 17650 relevant lines covered (90.72%)

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1 6091.1 438
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Source Files on build 6091