fix(persister): restore MergeableChanges handling in extractChangedTables

[yujonglee]

Summary

• Fix session folder (sessions/*.md, _meta.json, etc.) not persisting since nightly.8 release (worked in nightly.7)
• Restore MergeableChanges format handling in extractChangedTables()

Root Cause Analysis

Commit 64922dfba ("remove stamp repair that might cause dataloss") simplified extractChangedTables() too aggressively.

TinyBase sends changes in two different formats depending on the store type:

Format	Structure	When Used
Regular Changes	[changedTables, changedValues, 1]	Regular Store
MergeableChanges	[[changedTables, hlc?], [changedValues, hlc?], 1]	MergeableStore

The app uses a MergeableStore, which sends changes in format #2.

Before (nightly.7 - working)

const tablesOrStamp = changes[0];

// MergeableChanges: [[changedTables, hlc?], [changedValues, hlc?], 1]
if (Array.isArray(tablesOrStamp) && tablesOrStamp.length >= 1) {
  const tables = tablesOrStamp[0];
  if (tables && typeof tables === "object") {
    return unwrapMergeableTables(tables);
  }
  return null;
}

// Regular Changes: [changedTables, changedValues, 1]
if (tablesOrStamp && typeof tablesOrStamp === "object") {
  return tablesOrStamp as ChangedTables;
}

After (nightly.8 - broken)

const tables = changes[0];
if (tables && typeof tables === "object") {
  return tables as ChangedTables;
}

The Problem

With MergeableChanges, changes[0] is [changedTables, hlc?] (an array), not the tables object directly.

Since arrays are objects in JavaScript, typeof tables === "object" returns true, and the function returns [changedTables, hlc] instead of just changedTables.

When getChangedSessionIds receives this array:

• It tries to access changedTables.sessions
• Arrays don't have a .sessions property → returns undefined
• Function thinks nothing changed → returns { operations: [] }
• Nothing gets saved to disk

Test plan

• All 304 persister tests pass
• Added new test cases for MergeableChanges format
• Manual verification with nightly build

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[devin-ai-integration]

Devin Review found 1 potential issue.

View issue and 2 additional flags in Devin Review.