Add static Disruptor dry-run with default impairment

[rishabh-slamcore]

Parent

Parent: #26

What to build

Add the first static Disruptor tracer bullet: test-farm disrupt <scenario> --interface <client-facing-nic> --dry-run accepts a Disruptor Scenario File with a default impairment, resolves every discovered device to that default policy, builds a typed tc plan object, and renders dry-run output without applying tc or changing interface state.

This slice should keep the existing invocation Scenario File contract unchanged and should use fake or injected discovery in tests so the path is verifiable without real mDNS.

Acceptance criteria

• A Disruptor Scenario File with impairment.default parses separately from the existing invocation Scenario File.
• The CLI exposes test-farm disrupt <scenario> --interface <client-facing-nic> --dry-run and requires --interface.
• Dry-run resolves all discovered devices to the default impairment policy.
• Dry-run produces a typed tc plan object and human-readable dry-run output without applying or clearing tc state.
• Unit tests cover valid and invalid default-only Disruptor Scenario File parsing.
• Integration-style tests cover scenario file to fake discovery to resolved default policies to typed plan object.
• Existing invocation scenario parsing tests remain valid.

Blocked by

None - can start immediately