feat(configs): operator-overridable C-Chain genesis via LUX_CCHAIN_GENESIS_FILE

Adds env-driven override at the lowest layer of the genesis loader so
downstream networks (e.g. <tenant> at chainId 8675312) can reuse a
stock lqd binary without forking configs/{network}/cchain.json.

Resolution order in loadEmbeddedGenesisWithDynamic:
  1. LUX_CCHAIN_GENESIS_FILE — absolute path to a JSON file. Read at
     boot, used verbatim as the C-Chain genesis bytes.
  2. embedded configs/{network}/cchain.json (the previous default,
     immutable per-network).

Net behaviour for stock callers is unchanged: env unset → embedded
default. The override only fires when the file exists and is readable;
a misconfigured path fails loud with a wrapped error so misconfiguration
isn't silently masked by the embedded fallback.

Pairs with luxfi/node@d0a248f5 (LUX_AUTOMINE_CCHAIN_GENESIS_PATH for the
single-node automine path). The two cover both startup modes lqd uses.

configs/configs.go

@@ -194,10 +194,25 @@ func loadEmbeddedGenesisWithDynamic(networkName string, dynamicPChain *genesis.P
 		}
 	}
 
-	// Load C-Chain genesis (always embedded, immutable)
-	cchainData, err := embeddedGenesis.ReadFile(filepath.Join(networkName, "cchain.json"))
-	if err != nil {
-		return nil, fmt.Errorf("failed to read cchain.json: %w", err)
+	// Load C-Chain genesis. Operator override:
+	//   LUX_CCHAIN_GENESIS_FILE — absolute path to a JSON file that
+	//   replaces the embedded cchain.json verbatim. Lets downstream
+	//   networks (Liquidity at chainId 8675312, etc.) reuse the lqd
+	//   binary without forking the embedded genesis tree.
+	//
+	// Unset → embedded default (per-network, immutable).
+	var cchainData []byte
+	if override := os.Getenv("LUX_CCHAIN_GENESIS_FILE"); override != "" {
+		body, ferr := os.ReadFile(override)
+		if ferr != nil {
+			return nil, fmt.Errorf("read LUX_CCHAIN_GENESIS_FILE=%q: %w", override, ferr)
+		}
+		cchainData = body
+	} else {
+		cchainData, err = embeddedGenesis.ReadFile(filepath.Join(networkName, "cchain.json"))
+		if err != nil {
+			return nil, fmt.Errorf("failed to read cchain.json: %w", err)
+		}
 	}
 
 	// Build combined genesis config