fix(db): item disappears during optimistic-to-synced transition

[freddybreitenstein]

Summary

Fixes #1017 — items disappear from collection during the optimistic → synced transition when onInsert syncs data back (e.g., Electric's txid handshake).

Root Cause

collection.insert() called commit() before recomputeOptimisticState(true). Since commit() synchronously enters mutationFn/onInsert, the item was not in optimisticUpserts when onInsert ran. Any code checking collection.has(key) inside onInsert — or any sync data delivered during onInsert — would not find the item.

// BEFORE (broken):
directOpTransaction.applyMutations(mutations)
directOpTransaction.commit()          // ← enters onInsert, item NOT in optimistic
state.transactions.set(...)
state.recomputeOptimisticState(true)  // ← too late

// AFTER (fixed):
directOpTransaction.applyMutations(mutations)
state.transactions.set(...)
state.recomputeOptimisticState(true)  // ← item in optimistic BEFORE commit
directOpTransaction.commit()          // ← enters onInsert, item IS visible

Impact

This bug causes items to disappear from live queries during the optimistic → synced transition. With Electric SQL on React Native (20-second long-poll), the item could be gone for up to 20 seconds. On web with SSE, it manifests as a brief flicker.

Test Plan

• Added regression test: verifies collection.has(key) is true at every checkpoint during the transition
• Verified fix eliminates the visibility gap
• Full test suite: no regressions (162 pre-existing failures unchanged)
• Tested in production app (Moi — React Native + Electric SQL) — items now appear instantly