environments.toml docs: examples fail to parse; snake_case network keys silently ignored, so dev falls back to testnet #600

Summary

The network examples in the Environments guide (docs/site/docs/environments.md) can't be used as written.

1. They don't parse. The multi-line inline tables (network = { … } spread over several lines) are invalid TOML 1.0:

error: ⛔ ️parsing environments.toml: TOML parse error at line 2, column 12
  |
2 | network = {
  |            ^
newlines are unsupported in inline tables, expected nothing

2. Their keys are silently ignored. The guide uses rpc_url, network_passphrase, rpc_headers and run_locally, but Network is #[serde(rename_all = "kebab-case")] with no deny_unknown_fields (env_toml.rs#L143-L151). Only rpc-url, network-passphrase, rpc-headers and run-locally are read. The template and the tutorial use kebab-case, which is why they work.

The effect of (2) is surprising. With every network field unset, to_network lets stellar-cli fall back to its default network, so a development environment silently targets testnet. build also prints the Docker success lines even though it started nothing (build/mod.rs#L105-L111):

ℹ️  Starting local Stellar Docker container...
✅ Local Stellar network is healthy and running.
ℹ️  Using network at https://soroban-testnet.stellar.org

It then funds the default account on testnet through friendbot and deploys the workspace contracts there.

Repro

⚠️ Step 3 really funds an account and deploys contracts on testnet.

1. stellar scaffold init demo --template react -p npm -y && cd demo
2. In environments.toml, switch the [development.network] keys to snake_case, as the guide shows:

   [development.network]
   rpc_url = "http://localhost:8000/rpc"
   network_passphrase = "Standalone Network ; February 2017"
   run_locally = true

3. stellar scaffold build --build-clients development

Expected

• The guide's examples parse and do what they describe.
• A network that can't be resolved is an error, not a fallback to testnet. An Error::MalformedNetwork variant already exists for exactly this case but is never constructed (clients.rs#L89-L92).
• The "Starting… / healthy" lines only print when a container was actually started and probed.

Suggested fix

• Rewrite the environments.md examples with kebab-case keys and [<env>.network] tables, as in the template.
• Either accept the snake_case spellings with #[serde(alias = "…")], or add #[serde(deny_unknown_fields)] to Network so typos fail loudly. The v2 schema plan in plan: scaffold.yml proposal #592 already rejects unknown keys.
• Wire up Error::MalformedNetwork: require name, or both rpc-url and network-passphrase, before handing off to stellar-cli.
• Move the two printer lines inside the branch that actually starts the container.

Acceptance criteria

• Pasting any environments.md example into a fresh project parses and behaves as documented.
• A [development.network] table with unknown or misspelled keys fails with a clear error and never touches testnet.

Related: #592, #159

Reproduced with stellar-scaffold-cli 0.0.27 (cargo binstall, aarch64-apple-darwin), invoked through stellar-cli 25.2.0 on macOS, without Docker.