Add structured TOML configuration for Stargate

Problem

Stargate currently exposes runtime configuration as a large flat set of CLI flags and environment variables. Related settings are difficult to reason about, and mode booleans can become inconsistent with the settings they control.

Desired behavior

• Add a --config-file PATH option that loads a structured TOML file.
• When --config-file is present, use the file as the complete source of configuration and ignore all legacy CLI and environment settings.
• Fail startup if the selected file cannot be read, parsed, or validated. Do not fall back to legacy settings.
• Keep the legacy CLI path temporarily and emit one deprecation warning when it is used.
• Group related settings into descriptive sections and use section presence to select optional behavior where practical.
• Reject unknown fields and invalid combinations.
• Resolve relative file paths from the configuration file's directory.
• Retain DNS lookup only for local Kubernetes pod discovery. Remove generic DNS-based Stargate discovery.
• Migrate the request-router Helm chart and benchmark launchers to pass only --config-file.

Acceptance criteria

• Direct and reverse transport configurations are represented by distinct TOML sections.
• Kubernetes pod discovery is enabled by the presence of its section and self-only discovery is selected by its absence.
• Explicit remote Watch URLs remain supported for cross-region discovery.
• The Helm chart renders a mounted TOML ConfigMap and no legacy Stargate CLI flags.
• Rust, chart, and Bazel tests cover precedence, validation, topology selection, and legacy fallback behavior.

Start by locating Stargate's current CLI and environment configuration parsing, then inspect the request-router Helm chart and benchmark launchers. Review the Rust, chart, and Bazel test areas mentioned in the acceptance criteria. Done means TOML-only configuration, validation and precedence behavior, discovery selection, migrated chart and launchers, and coverage for the listed cases.