Fix offline mutations data loss on restart

[KyleAMathews]

Summary

Fix mutation.changes being lost when offline transactions are persisted and restored. After app restart, sync functions that rely on mutation.changes for partial updates would receive an empty object {}, causing server writes to fail with missing data.

Root Cause

The changes field was never added to the serialization flow:

1. SerializedMutation interface was missing the changes field
2. serializeMutation() didn't serialize changes
3. deserializeMutation() hardcoded changes: {} with a misleading comment "Will be recalculated"

The comment was wrong—changes was never recalculated anywhere. This meant any mutation function using the common pattern api.update(id, mutation.changes) would send empty updates after restart.

Approach

Add changes to the serialization round-trip, mirroring how modified and original are already handled:

// Serialize
changes: this.serializeValue(mutation.changes),

// Deserialize (with legacy data fallback)
changes: data.changes !== undefined 
  ? this.deserializeValue(data.changes) 
  : {},

The legacy fallback handles data serialized before this fix—without it, deserializeValue(undefined) would return undefined, causing TypeError when downstream code accesses properties on changes.

Key Invariants

• mutation.changes must survive the serialize → persist → deserialize cycle unchanged
• Legacy data (missing changes field) must deserialize without errors

Non-goals

• Not adding explicit version migration—the fallback to {} matches the previous (broken) behavior, so existing data continues to work as before
• Not refactoring the broader silent-failure patterns in OutboxManager—that's pre-existing and out of scope

Verification

cd packages/offline-transactions && pnpm test

All 26 tests pass. The existing "replays persisted transactions on startup" e2e test exercises this code path.

Files Changed

File	Change
types.ts	Add changes: any to SerializedMutation interface
TransactionSerializer.ts	Serialize/deserialize changes field with legacy fallback
TransactionSerializer.ts	Code quality: safer hasOwnProperty, unused param prefixes

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🦋 Changeset detected

Latest commit: 5e750bb

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@tanstack/offline-transactions	Patch

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commit: 5e750bb

[github-actions]

Size Change: 0 B

Total Size: 90.5 kB

ℹ️ View Unchanged

Filename	Size
./packages/db/dist/esm/collection/change-events.js	1.39 kB
./packages/db/dist/esm/collection/changes.js	1.19 kB
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./packages/db/dist/esm/utils/type-guards.js	157 B

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[github-actions]

Size Change: 0 B

Total Size: 3.47 kB

ℹ️ View Unchanged

Filename	Size
./packages/react-db/dist/esm/index.js	225 B
./packages/react-db/dist/esm/useLiveInfiniteQuery.js	1.17 kB
./packages/react-db/dist/esm/useLiveQuery.js	1.12 kB
./packages/react-db/dist/esm/useLiveSuspenseQuery.js	559 B
./packages/react-db/dist/esm/usePacedMutations.js	401 B

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