cardano-testnet: Replace Defaults.defaultXxxDir path-building with typed EnvLayout

[palas]

Problem

File paths within a testnet environment are constructed by concatenating strings at each call site, using functions like Defaults.defaultNodeDataDir, Defaults.defaultSpoKeysDir, Defaults.defaultGenesisFilepath, etc. The same path patterns are repeated independently in Testnet/Start/Cardano.hs, Testnet/Components/Configuration.hs, and Testnet/Defaults.hs.

This means:

• Renaming a directory requires finding and updating every string concatenation site.
• Typos in path construction (e.g., "nodedata" vs "node-data") are not caught by the compiler.
• There is no way to validate that all expected paths exist when loading an env.

Proposed fix

Define a typed EnvLayout data structure that represents the full directory structure:

data EnvLayout = EnvLayout
  { elRoot          :: FilePath
  , elConfigFile    :: FilePath
  , elGenesisFile   :: forall era. CardanoEra era -> FilePath
  , elNodeLayout    :: Int -> NodeLayout
  , elPoolKeys      :: Int -> PoolKeysLayout
  , elUtxoKeys      :: Int -> UtxoKeysLayout
  }

data NodeLayout = NodeLayout
  { nlDataDir    :: FilePath
  , nlTopology   :: FilePath
  , nlDbDir      :: FilePath
  , nlPortFile   :: FilePath
  }

Construct this once from the root path (replacing scattered Defaults.* calls), then thread it through both phases.

Benefits:

• Rename a directory = change one function (mkEnvLayout).
• Path existence can be validated structurally when loading an env.
• All path references are typed accessors, not string concatenations.

Depends on

• cardano-testnet: Wrap env into opaque TestnetEnv type with smart constructors #6463 (opaque TestnetEnv — EnvLayout naturally lives inside TestnetEnv)

Type: Refactoring

Effort: Medium-high (~200 lines: new types, mkEnvLayout, update ~20 call sites)

Risk: Medium (many call sites to update, but each change is mechanical and compiler-guided)