fix: preserve tuple index labels in lookups

[pesap]

Summary

• stop coercing bound index identifiers from string to number during compile-time index resolution
• preserve tuple-domain labels like 1.0 so variable instance lookup keys stay consistent
• add regression test for nodal-style tuple indices with numeric-like labels

Why

Numeric-like tuple labels were being normalized (1.0 -> 1) in expression lookup, which broke variable instance resolution and caused constraints not to materialize.

Validation

• cargo fmt
• cargo test -p arco-kdl --test compile_suite lowering_preserves_numeric_tuple_index_labels_in_expression_lookups
• cargo test -p arco-kdl --test compile_suite lowering_example_nodal_allocation_preserves_sparse_tuple_membership
• cargo run -- run ~/work/nodal-allocation/nodal_arco/input_debug.kdl