Add preview_correction_impact workflow proposal with explicit rule-based rationale

[akgohain]

Motivation

• Provide a deterministic, auditable pre-retraining signal that summarizes recent correction activity and export events to guide readiness decisions.
• Make the recommendation logic transparent and rule-based with an explicit, discoverable threshold constant.
• Ensure the helper is pure (no direct state mutation) so it can be used safely for previews and automated checks.

Description

• Added server_api/workflow/correction_preview.py which defines PREVIEW_CORRECTION_IMPACT_PROPOSAL_TYPE, explicit event type sets (RECENT_CORRECTION_EVENT_TYPES, RECENT_EXPORT_EVENT_TYPES) and the constant CORRECTION_RATE_PROCEED_THRESHOLD, plus build_preview_correction_impact_proposal(events) that returns a payload containing summary, recommendation, and a rule-based rationale.
• The proposal payload includes summary (total events, correction-related events, recent exports, correction_rate), recommendation (proceed or continue_proofreading), and rationale with rule, threshold, computed values, and a human-readable explanation showing the computation.
• Exposed the helper via server_api/workflow/__init__.py for package-level imports.
• Added tests/test_workflow_correction_preview.py with two scenarios validating low-correction -> proceed and high-correction -> continue_proofreading, and asserting no mutation of the input events list.

Testing

• Ran python -m pytest -q tests/test_workflow_correction_preview.py and observed 2 passed (both tests succeeded).
• Attempted the requested uv run pytest -q tests/test_workflow_correction_preview.py but it failed in this environment due to an unrelated editable dependency packaging issue for pytorch-connectomics (package metadata generation error).
• Tests validate summary fields, recommendation, rationale structure, threshold inclusion, and that the input events list is not mutated.

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Codex Task

[akgohain]

Consolidated into draft PR #165 (feat/codex-wave-integration). Closing this source draft to keep review in one place.