Support persistent default listen-addr for spacetime start

[0monish]

Description of Changes

This change allows spacetime start to use a project-configured default listen address from spacetime.json.

Behavior precedence is now:

1. Explicit CLI flag, such as spacetime start --listen-addr 127.0.0.1:5000
2. listen-addr from spacetime.json
3. Built-in default 0.0.0.0:3000

Example:

{
  "listen-addr": "0.0.0.0:4000"
}

With that config in place, spacetime start will start on 0.0.0.0:4000 unless --listen-addr is passed explicitly.

Implementation details:

• spacetime start now checks project config before forwarding to the standalone binary.
• Config lookup uses the existing spacetime.json discovery behavior, including parent-directory search and spacetime.local.json layering.
• The forwarded argument path preserves CLI precedence over config.
• Added unit tests covering detection of explicit --listen-addr and -l forms so config is only injected when the user did not already provide a listen address.
• Updated standalone config documentation to describe the new listen-addr behavior and precedence.

This is backward compatible:

• users without config still get 0.0.0.0:3000
• existing scripts that pass --listen-addr continue to behave the same
• the feature is project-local and works with the existing config-driven workflow

API and ABI breaking changes

None.

This change does not modify any public API or ABI. It only adds an additional configuration source for an existing CLI option while preserving current defaults and CLI override behavior.

Expected complexity level and risk

Complexity: 2/5

This is a small, localized CLI behavior change.

The main interaction point is config precedence between forwarded CLI arguments and project configuration. The implementation is low risk because it:

• preserves the existing standalone default when config is absent
• preserves explicit CLI --listen-addr and -l precedence
• reuses the existing spacetime.json discovery and layering behavior rather than introducing a new config system

The main thing reviewers may want to sanity-check is that the new config-backed behavior only applies when the user did not explicitly pass a listen address.

Testing

Completed:

• cargo fmt
• cargo check -p spacetimedb-cli
• cargo test -p spacetimedb-cli subcommands::start::tests
• cargo clippy -p spacetimedb-cli --all-targets

Reviewer checks:

• Run spacetime start without spacetime.json and confirm it still binds to 0.0.0.0:3000
• Add "listen-addr": "0.0.0.0:4000" to spacetime.json, run spacetime start, and confirm it binds to 0.0.0.0:4000
• Run spacetime start --listen-addr 127.0.0.1:5000 with config present and confirm the CLI flag still wins

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