feat(mutation): per-operation score breakdown and --min-score CI gate

[yuchou87]

Summary

• Per-operation mutation score (operation_scores in JSON, ## Per-Operation Mutation Score table in Markdown and HTML): groups results by source.spec_path from index.json, aggregates killed/survivors per API operation, sorts weakest-first. Badges: ⚠ below 70%, ✓ at 100%; HTML rows color-coded (light red / light green).
• --min-score CI gate: float64 flag (0.0–1.0, default 0 = disabled); exits 6 with a red violation list if any operation scores below the threshold. Validation rejects values outside [0, 1] before the run starts.
• No breaking changes: Operation field in CaseMutationResult is additive; OperationScores in MutationRun uses omitempty.

New files

• internal/mutation/score.go — ComputeOperationScores
• internal/mutation/score_test.go — 4 unit tests (basic, empty, unknown fallback, tiebreak)

Modified files

• internal/mutation/types.go — OperationScore type, CaseMutationResult.Operation, MutationRun.OperationScores
• internal/mutation/engine.go — reads source.spec_path, populates Operation, calls ComputeOperationScores
• internal/mutation/render.go — renderOperationTable (Markdown), renderOperationTableHTML (HTML)
• internal/mutation/render_test.go — 3 new render tests
• cmd/mutate.go — --min-score flag, validation, gate logic
• scripts/acceptance.sh — AT-410–AT-416

Test Plan

• go test ./... -count=1 — all packages pass
• bash scripts/acceptance.sh — 272/272 passed
• AT-410: mutate --help still shows --cases flag
• AT-411: mutation-report.md contains Per-Operation after a run
• AT-412: mutation-report.json contains operation_scores
• AT-413: --min-score appears in mutate --help
• AT-414: --min-score 1.5 returns error before running
• AT-415: --min-score 0 (default) does not change exit behavior
• AT-416: --min-score 0.99 exits 6 and prints violation list when an operation scores below threshold