stillwater-sc / universal / 26321362299

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feat(elreal): Phase 2 -- ZBCL<FpType> lazy co-list (#935)

* feat(elreal): Phase 2 -- ZBCL<FpType> lazy co-list of blocks

Phase 2 of the McCleeary LFPERA reimplementation (epic #923). Lands the
lazy stream type that holds an exact-real value as a co-list of
block<FpType> satisfying the 0-overlap invariant.

What ZBCL<FpType> is
- Empty co-list represents the real number 0.
- Non-empty co-list = (head block, tail thunk). The tail thunk is
  std::function<ZBCL()>; forced on first call to tail() or take(n) and
  memoised on the node. Nodes are shared via std::shared_ptr so ZBCL
  values are cheap to copy.
- Sentinel for end-of-stream is an empty thunk. The terminal block's
  tail() returns an empty ZBCL.

API
- cons(head, tail_thunk) / cons(head, materialised_tail) / singleton(b)
- head(), tail(), take(n), is_empty(), operator bool()
- empty<FpType>(), from_native<FpType>(double), to_double_approx(s, n)

0-overlap invariant
- Enforced by an assertion in tail(): when the thunk fires, if the
  resulting stream is non-empty, the head we hold and the new head must
  satisfy zero_overlap(this->head(), tail.head()). Debug-build only
  because evaluating the thunk to check would defeat laziness in
  release.

Cauchy-sequence proof obligation documented inline at the top of
zbcl.hpp. Concrete proofs accompany each operation in later phases.

Files
- include/sw/universal/number/elreal/zbcl.hpp: the lazy stream.
- include/sw/universal/number/elreal/zbcl_helpers.hpp: empty,
  from_native, to_double_approx.
- include/sw/universal/number/elreal/elreal_fwd.hpp: ZBCL fwd-decl.
- include/sw/universal/number/elreal/elreal.hpp: umbrella include.
- elastic/elreal/zbcl/cons_take.cpp: head/tail/take/is_empty contract.
- elastic/elreal/zbcl/laziness.cpp: atomic-counter test verifying take(n)
  forces exactly the thunks needed (take(1) -> 0 forces; take(2) -> 1;
  take(3) -> 2; memoisation prevents re-invocation).
- elastic/elreal/zbcl/from_native.cpp: empty + from_native + r... (continued)

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