fix(cli): replace process::exit with error return for --max-weight

[rocketman-code]

Closes #148

Summary

• Add MaxWeightExceeded variant to Error enum with structured fields (kind, weight, module_count, threshold)
• Replace process::exit(1) in run_trace with Err(Error::MaxWeightExceeded { .. }) so the error flows through the centralized error handler in main()
• The Display impl formats the same user-facing message as before, so CLI output is unchanged

Test plan

• cargo test --workspace passes (all 338 tests)
• cargo clippy --workspace --all-targets -- -D warnings clean
• cargo fmt --check clean

[rocketman-code]

Closing: patch contents already on main. Branch has zero diff after rebase.