CommonSubexpressionElimination

Common Subexpression Elimination (CSE) for UPLC terms.

Identifies duplicated subexpressions and hoists them into shared let-bindings (encoded as Apply(LamAbs(name, body), expr)). This reduces on-chain execution budget by avoiding redundant computation.

==Algorithm==

Adapted from the Plutus CSE (3-pass path-based algorithm):

1. '''Count pass''': Traverse the term, assigning unique path IDs at evaluation boundaries (LamAbs, Delay, Case branches). Count (TermKey, Path) -> occurrences pairs. Track which variables are introduced at each path segment.
2. '''Merge & filter''': Merge counts to common ancestor paths and filter to candidates with total count >= 2 that are not work-free. Verify that all free variables of the candidate are in scope at the bind path.
3. '''Substitute pass''': For each candidate (largest first), replace occurrences with a fresh variable and insert Apply(LamAbs(freshName, body), candidate) at the bind point.

==Safety==

• Work-free terms (Var, Const, LamAbs, Delay, Builtin, unsaturated builtins) are never extracted
• Error terms are never extracted
• Bare Force(Builtin(...)) patterns are skipped (handled by ForcedBuiltinsExtractor)
• CSE does not hoist expressions past lambdas that bind their free variables
• CSE does not hoist across evaluation boundaries, preserving evaluation order