Engine: commit revision on write replies (EN-3) #256

Goal. Every committed write reply carries the revision it committed at (result.revision), so callers record the commit revision without a second query.

Why. Decision records need revision_commit, and every API write should return the revision it produced. Today a write reply carries no revision.

Done when. Fact, update, rule and program commits report revision in the reply; refused or no-op programs report the revision they were evaluated at; both SDKs expose it; covered by protocol tests.

Plan reference: EN-3. Part of the Engine production readiness milestone (the current focus; tracker #199). Rule for every engine PR: millisecond query and update latency must not regress, and the perf gate runs before the PR opens.