Fix consLazy laziness: tail must not be forced when accessing head

[Copilot]

consLazy was wrapping both the head cell construction and tail evaluation inside a single lzy, meaning forcing the head cell (e.g. via LazyList.head) also forced the lazy tail — defeating the purpose of consLazy.

Changes

• LazyList.fs — Rewrite consLazy to use notlazy for the outer cell so the head is immediately available, with only the tail wrapped in a deferred lzy:

  // Before: forces tail whenever head cell is evaluated
  let consLazy x (l: Lazy<LazyList<'T>>) =
      lzy(fun () -> consc x l.Value)
  
  // After: head is immediate; tail is lazy
  let consLazy x (l: Lazy<LazyList<'T>>) =
      notlazy(CellCons(x, lzy(fun () -> force l.Value)))

• LazyListTests.fs — Replace the failwith-based divergence test with a mutable tailForced flag to explicitly assert that neither construction nor LazyList.head forces the tail:

  let mutable tailForced = false
  let tail = lazy (tailForced <- true; LazyList.ofList [ 2; 3 ])
  let ll = LazyList.consLazy 1 tail
  Expect.isFalse "construction should not force the tail" tailForced
  let _ = LazyList.head ll
  Expect.isFalse "head should not force the tail" tailForced

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