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

## 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: ```haskell 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 - #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)