Add generate-config CLI command for GNSS degradation config files

[Copilot]

Users need a way to create properly formatted GNSS degradation config files without manually constructing the schema. This adds a generate-config subcommand that outputs template configs in JSON, YAML, or TOML.

Changes

Core (core/src/messages.rs)

• Added Default impl for GnssFaultModel (→ None) and GnssDegradationConfig (passthrough scheduler, no faults, seed=42)
• Added #[serde(default)] annotations for partial config deserialization
• Added roundtrip test for default config serialization

CLI (sim/src/main.rs)

• Added generate-config subcommand with -o/--output path argument
• Format auto-detected from file extension (.json/.yaml/.yml/.toml)
• Refactored to use #[command(flatten)] for SimArgs composition
• Fixed dead logic in validate_output_path

Usage

strapdown-sim generate-config -o config.yaml

Output:

scheduler:
  kind: pass_through
fault:
  kind: none
seed: 42

Use with simulations via --config:

strapdown-sim closed-loop -i data.csv -o out.csv --config config.yaml

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add option to generate config files via cli</issue_title>
<issue_description>We have functionality hooks for reading in input configurations in .json, .yaml, etc. but that doesn't necessarily mean the input is standardized in the correct format. I'd like a way to be able to walk the user through creating a config file by creating a blank template for them.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add option to generate config files via cli #110

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