Engine: structured per-statement counts in write replies (EN-4) #257

Goal. Write replies report per-statement counts as structured data instead of message text.

Why. Callers (the gateway, both SDKs) must know whether a guarded update's token landed. Today they parse Update: N deleted, M inserted. text, and a program ending with a query loses the counts entirely.

In flight. PR #255 (draft): statements: [{index, kind, inserted, deleted}] on write replies.

Done when. #255 merges; the SDK compilers read counts from the field and drop the text grammar.

Plan reference: EN-4. 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.