Evidence gate: piece completion only on typed promise backed by artifacts, never a bare claim

[wesleysimplicio]

Parent epic: #64

Goal

Port the simplicio-loop driver rule — stop only on an evidence-gated <promise>, ignore a bare promise — to piece completion. A piece transitions to done/scheduled only when the completion claim is backed by machine-checkable artifacts on disk; a textual "looks good, done" is ignored.

Required evidence per piece (from CLAUDE.md DoD)

• manifest.json present under outputs/<client>/<date>/<piece-id>/ and schema-valid.
• Compliance JSON with pass: true (or a logged human override event).
• qa-tech-specs report pass.
• 4-platform caption set files present.
• Playwright evidence artifact recorded (e2e/ run reference or screenshot/video path) — reuse the existing watcher_report_path plumbing from the watcher-gate.
• Run rows appended to data/llm-usage.jsonl and data/runs.jsonl.

Scope

• evidence gate <piece-id> worker: verifies every item above against the filesystem, exits non-zero with the missing-item list otherwise.
• Wire it as the single transition guard used by lib/cli/generate.ts (draft→scheduled) and by the anchor gate (Anchor: freeze campaign/piece acceptance criteria at intake and gate "done" on verified AC #65) — one implementation, no duplicated checks.
• Missing toolchain (Playwright/FFmpeg not installed) → blocked, never a fake pass — same posture as scripts/video_evidence.py in simplicio-loop.

Acceptance criteria

• E2E test: a piece with all artifacts passes; deleting any single artifact makes the same gate fail with that item named.
• E2E test: a completion claim with no artifacts is ignored (piece stays in its current state).
• Gate output is JSON ({pass, missing[], evidence_paths[]}) so the journal (Journal: durable run-journal + stall detector so the loop switches strategy instead of oscillating #66) and PR evidence builder can consume it.