Prefix per-job log lines under --render-concurrency

[jbrecht]

From the code-reviewer / performance-engineer notes on PR #64. Severity: LOW (UX polish).

Under --render-concurrency > 1, each job's ▶ start / ✓ done lines interleave on stdout with no per-line job identifier. Readable at N=2; hard to follow at higher N.

Suggestion

When renderConcurrency > 1, prefix each log line with the tutorial id (+lang), e.g. [getting-started.es] ✓ …, so interleaved output stays traceable. Leave single-render (N=1) output exactly as-is.

Acceptance

• Concurrent batch logs are unambiguously attributable to a job; serial output unchanged.