fix(native): allow virgin empty chat replay

[kvkenyon]

Summary

• allow cursor-zero replay for an unbound virgin conversation as an empty snapshot
• retain binding, scope, and nonzero cursor enforcement
• add SQLite and coordinator regressions for empty replay and rejection cases

Validation

• npm run check (passed)
• scripts/native-check.sh (CLT-safe checks passed; XCTest remains for full Xcode)
• focused Swift XCTest target unavailable under Command Line Tools (no XCTest module)