feat: expose terminal failure locations

[andreatnvidia]

📋 Summary

Add an opt-in, lightweight terminal-failure locator for callers that need to reconcile omitted rows without enabling full task tracing. This supports merged Anonymizer pipelines while leaving the default generation path unchanged.

🔗 Related Issue

Closes #860

🔄 Changes

• Add capture_terminal_failures to create(), acreate(), and preview().
• Return typed (seed_row_index, column) terminal failures on successful results and zero-row generation errors.
• Record only failures that remain terminal after retry and salvage, excluding skipped, recovered, downstream-unrun, and early-shutdown-cancelled tasks.
• Expand batch failures to affected global row positions and scope resume results to the current invocation.
• Expose early_shutdown on successful results so callers can detect when cancelled rows have no truthful column attribution.
• Keep collection disabled by default and retain only O(number of terminal failures) records when enabled.

🔍 Attention Areas

⚠️ Reviewers: Please pay special attention to the public API contract and early-shutdown behavior.

• seed_row_index is the requested generation-sequence position. It maps directly to the raw seed row only for compatible ordered seeding.
• terminal_failures excludes rows cancelled by global early shutdown. Callers should check early_shutdown before treating the list as complete.

🧪 Testing

• Config suite: 644 passed
• Engine suite: 2,262 passed
• Interface suite: 1,121 passed, 1 skipped
• Unit tests added and updated
• E2E tests: N/A - no external provider behavior changed

✅ Checklist

• Follows commit message conventions
• Commits are signed off (DCO)
• Architecture docs updated (N/A - no architectural change)

[github-actions]

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