feat(apps): add deployment preflight and journal engine

[bnema]

P4 preparation slice 1 (unreachable). Captured-revision preflight:

digest/secret/volume/reservation/resource gates, op journal, targeted

convergence refusal, terminal outcomes. 8 tests. Self-reviewed.

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[bnema]

Consolidated into #276 (single v2.50 preparation PR, same content). Closing the stack.