feat(FN-3390): export eval score category type and harden evaluator switch

Exported the eval score category type from `@fusion/core` and added a defensive guard in the evaluator to prevent edge-case failures in the score evaluation switch.

Fusion-Task-Id: FN-3390

docs/evals.md

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+# Task Evaluations Scoring Contract
+
+[← Docs index](./README.md)
+
+## Overview
+
+Fusion task evaluations use one canonical 0–100 integer scoring system for three categories: `agentPerformance`, `taskOutcomeQuality`, and `processCompliance`.
+
+Authoritative score math lives in `packages/core/src/eval-scoring.ts`. AI output is advisory input only.
+
+## Category Definitions
+
+- **agentPerformance**: execution effectiveness, recovery from issues, and quality of agent decision-making.
+- **taskOutcomeQuality**: correctness, completeness, verification quality, and shipped-result quality.
+- **processCompliance**: adherence to required workflow steps (tests, docs, review/merge expectations, commit/task conventions).
+
+## Score Bands
+
+- `0..39` → `failing`
+- `40..59` → `weak`
+- `60..74` → `acceptable`
+- `75..89` → `strong`
+- `90..100` → `excellent`
+
+## Overall Formula
+
+Category weights:
+
+- `agentPerformance`: `0.30`
+- `taskOutcomeQuality`: `0.45`
+- `processCompliance`: `0.25`
+
+Overall score formula:
+
+`overallScore = round(sum(category.finalScore * category.weight))`
+
+All scores are clamped/validated to integer `0..100`.
+
+## Deterministic vs AI Blend
+
+Per-category final score:
+
+`finalScore = round(clamp((deterministicScore * 0.7) + (aiScore * 0.3), 0, 100))`
+
+Authority flow:
+
+1. deterministic signal collection produces per-category deterministic inputs.
+2. AI evaluator emits `aiScore`, rationale, and evidence for each category.
+3. Core scoring helpers compute authoritative `finalScore` and `overallScore`.
+4. Eval store persists both the structured breakdown and computed overall.
+
+## Persisted Score Payload
+
+Each `eval_task_results.categoryScores[]` item stores:
+
+- `category`
+- `deterministicScore`
+- `aiScore`
+- `finalScore` (authoritative)
+- `weight`
+- `band`
+- `rationale`
+- `evidence[]`
+
+`overallScore` is authoritative only when derived from these category finals using `computeOverallScore`.
+
+## Non-Goals
+
+This contract does not define:
+
+- follow-up task creation policy
+- eval settings UX
+- eval dashboard/list rendering
+- exhaustive cross-source evidence harvesting