Replace cabal test override with cooked-test commands

[mmontin]

Closes #402.

Problem

The default dev shell redefined cabal as an exported bash function in its shellHook to strip the verbose HPC Writing: ….html output of cabal test (coverage was enabled by default in cabal.project). That function was exported with export -f, so it was not inherited under direnv with shells other than bash (e.g. zsh), and the noisy output reappeared.

Changes

• cabal.project: remove the default coverage settings (the redundant coverage + deprecated library-coverage fields). Coverage is now opt-in, so plain cabal test output is clean.

• flake.nix: replace the fragile cabal function override with two real scripts on PATH in the default shell (work regardless of shell/direnv):

  • cooked-test — runs the suite without coverage.
  • cooked-test-coverage — runs with --enable-coverage and filters the verbose Writing: ….html lines (preserving the real test exit status via PIPESTATUS).

  Both remove a stale spec.tix first to avoid HPC .tix/.mix hash mismatches (-fignore-hpc-changes lets GHC reuse instrumented objects across runs), and forward extra arguments to the tasty test binary (e.g. cooked-test -p "Balancing").

• Also removed a duplicate pkgs.lmdb entry in LD_LIBRARY_PATH.

• CONTRIBUTING.md / CHANGELOG.md: documented the new commands and the default-coverage removal.

Validation

• Pre-commit nixfmt hook passes; nix eval of the default dev shell succeeds.
• Verified cooked-test / cooked-test-coverage both run, including with a -p pattern, and that alternating between them no longer triggers the HPC .tix mismatch.