cardano-testnet: Add TestnetEnvManifest to systematise cross-phase data passing

[palas]

Problem

The interface between createTestnetEnv (step 1) and cardanoTestnet (step 2) is entirely implicit — it is a directory layout with naming conventions scattered across Defaults.hs, Configuration.hs, and Cardano.hs. Critical information like node count and types is not persisted, forcing the user to re-specify CLI options consistently across both invocations. Other data like testnetMagic must be re-parsed from genesis files.

This makes the cross-phase contract fragile: adding a new file in step 1 and forgetting to read it in step 2 is not caught by the compiler.

Proposed fix

Define a TestnetEnvManifest type that captures everything step 1 produces that step 2 needs:

data TestnetEnvManifest = TestnetEnvManifest
  { manifestNodes        :: NonEmpty NodeManifestEntry
  , manifestUtxoKeys     :: Int
  , manifestTestnetMagic :: Int
  , manifestCreatedAt    :: UTCTime
  , manifestEra          :: AnyShelleyBasedEra
  } deriving (Eq, Show, Generic)
    deriving anyclass (ToJSON, FromJSON)

data NodeManifestEntry = NodeManifestEntry
  { manifestNodeId   :: Int
  , manifestNodeType :: NodeType  -- SPO | Relay
  } deriving (Eq, Show, Generic)
    deriving anyclass (ToJSON, FromJSON)

• createTestnetEnv writes testnet-env.json as its final step.
• cardanoTestnet reads it as its first step when running from a pre-existing env.
• In the direct cardano path, the manifest is constructed in-memory and passed directly (no disk round-trip needed).

The manifest file is human-readable, so users can inspect what create-env produced.

Adding a new cross-phase datum = adding a field to TestnetEnvManifest -> compiler forces both sides to update.

Depends on

• cardano-testnet: Split CardanoTestnetOptions into EnvCreationOptions and RuntimeOptions #6459 (split options records — so the manifest carries EnvCreationOptions fields cleanly)

Type: Refactoring

Effort: Medium (~60 lines: type definition, ToJSON/FromJSON, write in createTestnetEnv, read in cardanoTestnet)

Risk: Low-medium (new serialisation code; failure mode is a clear parse error, not silent breakage)