perf: eliminate double serialization in event append path

[jwilger-ai-bot]

Problem

Events are serialized twice during execute():

1. StreamWrites::append() (eventcore-types/src/store.rs:89) calls serde_json::to_value(&event) — produces a serde_json::Value
2. Each backend's append_events() then calls serde_json::to_string(event_data) to convert the Value to a JSON string for SQL binding

At ~78 ns per serialize pass plus to_value overhead, this adds ~150-200 ns per event unnecessarily. For a 100-event batch, that's ~15-20 µs of waste.

Proposed Solution

Options (in order of preference):

1. Serialize directly to String in StreamWrites::append() and store the string, not the Value. Backends bind the string directly.
2. Have backends accept the Value and use serde_json::to_writer to a pre-allocated buffer instead of to_string.

Option 1 is simpler and eliminates the intermediate Value allocation entirely.

Expected Impact

~150 ns/event saved across all backends. Most impactful for high-throughput in-memory and SQLite workloads.

Location

• eventcore-types/src/store.rs — StreamWrites::append() and StreamWriteEntry struct
• eventcore-postgres/src/lib.rs — append_events() SQL binding
• eventcore-sqlite/src/lib.rs — append_events() SQL binding

Benchmark Baseline

Run cargo bench -p eventcore-bench --bench store_operations -- 'store/append' and cargo bench -p eventcore-bench --bench serialization to measure before/after.