fix: harden identity and legacy schema upgrades

[dannyshmueli]

Summary

• make OSS writes identity-aware at SQL time so late /track events still resolve to the canonical user after /identify
• add legacy schema compatibility for both SQLite and Cloudflare D1, including events.country and identity_map
• cover the upgrade and race cases with end-to-end regression tests and document the trustworthy /query surface

Verification

• npm test
• npm run lint

Context

• Part of AGE-2

[dannyshmueli]

Closing this as superseded.

The underlying identity/schema issue was real, but the right fix ended up spanning @agent-analytics/core, this OSS repo, and agent-analytics-hosted so late /track writes canonicalize through identity_map everywhere instead of only in OSS.

This PR also mixed in /query contract changes that did not line up with the pinned core behavior, so I don't want to merge it as the source of truth.

The problem this PR was trying to solve is now handled in the shared/core path plus both consumers, with tests passing in all three repos.