Cardano Updates

builtins.appendContext validates that store paths exist locally, which
fails when paths come from flake inputs that aren't available on the
evaluating machine (e.g. CHaP in cardano-node builds).

Instead, preserve string context through parsing by using
builtins.substring 0 0 to capture context from input strings and
re-attach it via concatenation after builtins.match/split strip it.

- parseBlockLines: carry context from input lines to parsed attr values
- parseSourceRepositoryPackages: carry context through builtins.split
- extractSourceRepoPackageData: use unsafeDiscardStringContext for
  sha256map attribute name lookups
- parseRepositoryBlock: remove addContext helper that used appendContext,
  use attrs.url directly (context now preserved by parseBlockLines)
- call-cabal-project-to-nix: remove appendContext on writeText, export
  rawCabalProjectContext for use by load-cabal-plan
- load-cabal-plan: use rawCabalProjectContext concatenation instead of
  appendContext

Add unit tests verifying context flows through parseBlockLines and
parseSourceRepositoryPackages.