Select a scenario configuration at startup, with a ready-to-run development.toml

[jwrosewell]

User story

As a developer on a standard machine, I want to select a ready-to-run configuration at startup, so that I can run the native dev server without an edge compute provider or build-time config edits.

Description

Application settings are baked into the binary at build time from trusted-server.toml. Add a native runtime override. When the TRUSTED_SERVER_CONFIG environment variable names a config file, a native build reads and validates it at startup instead of the baked configuration. The Fastly and Cloudflare wasm targets cannot read files at runtime and keep the build-time configuration. Add a committed development.toml for a standard developer machine. It is vanilla, with no edge compute provider and real local-development secrets, so the dev server runs as-is, unlike trusted-server.toml which ships placeholder secrets that must be overridden.

Done when

• On native builds, TRUSTED_SERVER_CONFIG selects a config file that is read and validated at startup, failing loudly on a bad path.
• The wasm targets keep the build-time configuration.
• A committed development.toml parses, validates, and uses real local-development secrets.

References

Part of #777. Relates to #716 (Seamless Publisher Experience for Deploying and Testing).