FloatIn: an IR-level replacement for pushDeclarationsDownTheInnerScope - #151

Closes #136.

Summary

pushDeclarationsDownTheInnerScope (a Lua-AST rule, disabled in #137) sank a let-bound declaration into the body of the returned lambda. That moved evaluation from partial application into every call of the returned function, losing sharing and delaying errors until the function was actually invoked.

This PR replaces it with an IR-level pass, Language.PureScript.Backend.IR.FloatIn, that sinks a Let-bound value into the single IfThenElse branch that uses it. Because it operates on the IR, lambdas are visible as distinct Abs nodes rather than opaque closures, so the pass can rule out exactly the shape that broke sharing: a binding is never sunk across a lambda.

Three correctness issues surfaced during design and review, all covered by regression tests:

• A bare Let-to-Let transit is not itself progress. Without this rule, let y = a in let x = b in x + y would swap the two bindings on every run. A transit only counts once the descent past it crosses an IfThenElse boundary.
• A top-down driver is not enough. It decides an outer Let before its children are rewritten, so an inner sink that confines a name's uses to one branch unblocks the enclosing binding only on a second run (a live flake in the idempotence property, ~6% of generated expressions), and a fully-collapsing Let exposes its body to no further rewriting (the "Recurse-escape" class fixed for DCE in First-class passes for the IR pipeline: Pass values, invariant checks, deterministic name supply #149). The pass therefore rewrites bottom-up (transformOf subexpressions): one pass is complete and idempotent, checked by a property and by unit regressions.
• Sinking must stop at the branch root when the only uses sit inside a lambda in that branch: the binding lands above the Abs, never inside it. Pinned by a unit test and by the eval oracle below.

None of the existing golden fixtures happen to exercise the shape this pass targets (a multi-use, non-inlinable Let binding confined to one IfThenElse branch, since the ordinary inliner already handles the single-use case). This PR adds a small new golden (Golden.FloatIn) with a runnable eval oracle that observes the evaluation policy, not just the computed value: a tick foreign function prints a line every time it is evaluated, and the fixture routes all uses of the shared binding through a lambda called twice inside one branch. The pinned output proves the binding runs once inside the taken branch (sharing preserved, so the sink stopped above the lambda), and never on the branch not taken.

Changes

• lib/Language/PureScript/Backend/IR/FloatIn.hs (new): the pass. Free-reference tests use an early-exit occurrence scan (usesName, exact under the GUC) instead of building full countFreeRefs maps per query.
• lib/Language/PureScript/Backend/IR/Optimizer.hs: wires it in between the post-merge optimize/dce fixpoint and magicDo.
• test/Language/PureScript/Backend/IR/FloatIn/Spec.hs (new): 20 unit cases plus 4 property tests (GUC preservation, free-reference preservation, idempotence, bound-name preservation) against generated expressions. The idempotence property was additionally hammered offline on 20k generated expressions.
• test/Language/PureScript/Backend/IR/SpecUtils.hs (new) and a parametrized prop in Test.Hspec.Hedgehog.Extended: spec helpers shared with the DCE and FlattenDeepBinds specs instead of a third copy.
• test/ps/src/Golden/FloatIn/Test.purs (new, with .lua FFI) and its golden/eval fixtures.
• lib/Language/PureScript/Backend/Lua/Optimizer.hs: deletes the now-superseded pushDeclarationsDownTheInnerScope rule and its pinning test.

Test plan

• cabal test all --test-show-details=direct passes (365 examples, 0 failures)
• fourmolu -i lib/ exe/ test/ and hlint lib/ exe/ test/ clean
• New Golden.FloatIn eval oracle passes, pinning both the computed values and the single tick evaluation inside the taken branch
• Idempotence and free-reference preservation additionally hammered on 20000 generated expressions in GHCi: 0 violations
• All other golden fixtures unchanged (no existing fixture exercised this shape before)