Why: project channel convention (pseudo-key project:<projectId>) was
duplicated across runmgr/server/store; centralize so replay envelopes
match live stamping.
Evidence: go test green.
Impact: no behavior change; enables store replay stamping.
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项目流伪键约定收进 core 辅助函数（ProjectChannelKey/ProjectFromChannel），发布、路由、重放三处共用，保证重放信封与实时盖章一致。