feat: improve parallel output rendering with per-output progress bars and explicit tests

[Copilot]

Parallel output rendering via Rayon's par_iter() was already wired up, but called MultiProgress::add() inside the parallel closure — acquiring an internal mutex on every call and potentially serializing workers. This PR eliminates that contention and adds explicit validation of the parallel behavior.

Changes

• Pre-create per-output progress bars before the rayon section, then zip with outputs:

  let output_bars: Vec<ProgressBar> = config.outputs.iter()
      .map(|o| create_output_bar(&mp, &o.output_type.to_string()))
      .collect();
  
  config.outputs.par_iter()
      .zip(output_bars.par_iter())
      .map(|(output, bar)| { /* each worker owns its bar, no shared mutex */ })
      .collect()

• create_output_bar() helper extracts bar creation logic and documents the pre-creation rationale

• Named symbol constants (SYMBOL_OK, SYMBOL_FAIL, SYMBOL_SKIP) replace hardcoded emoji strings across all render status messages

• Three new tests covering the parallel execution acceptance criteria:

  • test_parallel_dry_run_produces_result_per_output — par_iter processes every configured output
  • test_parallel_failure_isolation_and_aggregation — each format fails independently; all failures aggregated into one error
  • test_parallel_renders_all_formats_independently — HTML + PDF + DOCX render concurrently and all output files are produced

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