Extract cardano-testnet-paths micro-package from cardano-testnet

## Context Part 1 of #6510. Pure refactoring — no behavior change. Several path conventions (node names, data dirs, socket paths, UTxO key paths) are currently defined as inline literals or local functions scattered across `cardano-testnet`. These paths form a contract that other packages (like `tx-generator`, see #6510) need to consume. Extracting them into a lightweight shared package enables compile-time safety for directory layout conventions. ## Changes ### New: `cardano-testnet/cardano-testnet-paths/cardano-testnet-paths.cabal` Micro-package cabal file. Depends only on `base` and `filepath`. ### New: `cardano-testnet/cardano-testnet-paths/src/Testnet/Paths.hs` **`Testnet.Paths`** exports: ```haskell defaultNodeName :: Int -> String -- "node" <> show n defaultNodeDataDir :: Int -> FilePath -- "node-data" </> defaultNodeName n defaultUtxoKeyDir :: Int -> FilePath -- "utxo-keys" </> "utxo" <> show n defaultUtxoSKeyPath :: Int -> FilePath -- defaultUtxoKeyDir n </> "utxo.skey" defaultUtxoVKeyPath :: Int -> FilePath -- defaultUtxoKeyDir n </> "utxo.vkey" defaultUtxoAddrPath :: Int -> FilePath -- defaultUtxoKeyDir n </> "utxo.addr" defaultSocketDir :: FilePath -- "socket" defaultSocketName :: FilePath -- "sock" defaultSocketPath :: Int -> FilePath -- defaultSocketDir </> defaultNodeName n </> defaultSocketName defaultConfigFile :: FilePath -- "configuration.yaml" defaultPortFile :: Int -> FilePath -- defaultNodeDataDir n </> "port" ``` ### Modify: `cardano-testnet/src/Testnet/Defaults.hs` Import and re-use `defaultNodeName`, `defaultNodeDataDir` from `Testnet.Paths` instead of defining locally. `defaultUtxoKeys` delegates to `defaultUtxoSKeyPath` and `defaultUtxoVKeyPath` for path construction. ### Modify: `cardano-testnet/src/Testnet/Runtime.hs` Import `defaultSocketDir`, `defaultSocketName` from `Testnet.Paths` instead of using bare `"socket"` and `"sock"` string literals. ### Modify: `cardano-testnet/src/Testnet/Filepath.hs` Import `defaultSocketDir` from `Testnet.Paths` instead of using bare `"socket"` string literal. ### Modify: `cardano-testnet/src/Testnet/Start/Cardano.hs` Import `defaultConfigFile` and `defaultUtxoAddrPath` from `Testnet.Paths` instead of using inline string literals. ### Modify: `cardano-testnet/cardano-testnet.cabal` Add dependency on `cardano-testnet-paths`. ### Modify: `cabal.project` Add `cardano-testnet/cardano-testnet-paths` to the `packages` list.