test: add property-based tests for parsers and resolvers

[rocketman-code]

Context

Parent: #160

The import parsers, resolvers, and flag parsers accept complex string inputs where edge cases are hard to enumerate manually. Property-based testing (proptest) can systematically explore the input space.

Expected Behavior

Proptest strategies that generate random but structured inputs and verify invariants hold.

Scope

• parse_flags: arbitrary flag sequences never panic, always return Ok or Err
• Command::parse: arbitrary strings never panic
• classify_diff_arg: arbitrary strings never panic (may return error)
• Import parsers (TS + Python): arbitrary source code never panics (may return empty imports)
• Resolvers: resolved paths are always canonical and within project root
• format_size: output is always parseable back to a number, monotonically increasing with input
• ModuleGraph: adding edges between valid NodeIds never panics