Cardano Updates

Add uplc-evaluator to exposed-haskell-packages in nix/outputs.nix to make
it available via nix build/run commands.

This allows users to easily build and use the uplc-evaluator service:
  nix build .#uplc-evaluator
  nix run .#uplc-evaluator -- --help

The executable provides an asynchronous file-based interface for
benchmarking UPLC programs, as specified in plutus-benchmark/uplc-evaluator/SPEC.md.

Add uplc-evaluator to exposed-haskell-packages in nix/outputs.nix to make
it available via nix build/run commands.

This allows users to easily build and use the uplc-evaluator service:
  nix build .#uplc-evaluator
  nix run .#uplc-evaluator -- --help

The executable provides an asynchronous file-based interface for
benchmarking UPLC programs, as specified in plutus-benchmark/uplc-evaluator/SPEC.md.

The test harness was searching for the uplc-evaluator executable only in
hardcoded dist-newstyle paths with specific GHC versions. This failed in
nix builds where the executable is made available via build-tool-depends
in PATH.

Solution: Use System.Directory.findExecutable to locate uplc-evaluator
in PATH. This works for both:
- cabal builds: cabal test adds build-tools to PATH
- nix builds: nix adds dependencies to PATH via build-tool-depends

This eliminates the need to maintain a list of hardcoded paths for
different platforms and GHC versions.

Document the variable capture bug fix in the changelog.

Add 8 test cases covering variable capture scenarios in dischargeCekValue:

- Free variables under 1, 2, and 3 lambdas

- Deeply indexed free variables

- Multiple free variables in the same term

- Nested environment structures

Tests verify that free variables are correctly shifted to prevent capture when terms are discharged from the evaluation environment.

Tests for #7526

Add shiftTermBy function to correctly shift free variables when discharging values from the environment. Previously, free variables in discharged terms could be captured by outer lambdas, causing incorrect variable references in the output term.

The fix tracks binding depth during discharge and shifts free variables (those with indices beyond the current binding depth) by the appropriate amount to maintain correct scoping.

Resolves #7526