New POST /block/volume/plan endpoint returns full placement preview:
resolved policy, ordered candidate list, selected primary/replicas,
and per-server rejection reasons with stable string constants.
Core design: evaluateBlockPlacement() is a pure function with no
registry/topology dependency. gatherPlacementCandidates() is the
single topology bridge point. Plan and create share the same planner —
parity contract is same ordered candidate list for same cluster state.
Create path refactored: uses evaluateBlockPlacement() instead of
PickServer(), iterates all candidates (no 3-retry cap), recomputes
replica order after primary fallback. rf_not_satisfiable severity
is durability-mode-aware (warning for best_effort, error for strict).
15 unit tests + 20 QA adversarial tests.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Ping QiuandClaude Opus 4.6
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