Do not silently overwrite partial blockchain MMR nodes in IndexedDB

The IndexedDB store writes partial blockchain MMR nodes with `bulkPut()`:

- `insertPartialBlockchainNodes()`
- `updatePartialBlockchainNodes()`

`bulkPut()` overwrites an existing row with the same key. This means that if the store already has an authentication node for some MMR index, a later write with the same index but a different value silently replaces it.

These nodes are part of the local `PartialMmr` state. If a buggy or malicious sync path ever tries to write a different value for an already-known node index, the web store should not silently replace the old value.

This is related to #190 and #104, but not the same issue. #190 is about rolling back `AccountSmtForest` after failed writes, while this issue is about rejecting conflicting writes to already-stored `PartialMmr` authentication nodes.

Expected behavior:

- writing the same node index with the same value should be accepted or ignored
- writing the same node index with a different value should return an error
- the old value should remain stored

Suggested fix: replace `bulkPut()` with insert-if-missing plus an equality check on conflicts.

Look at the IndexedDB store methods `insertPartialBlockchainNodes()` and `updatePartialBlockchainNodes()` that use `bulkPut()`. Understand the `PartialMmr` state and how authentication nodes are keyed by MMR index. Replace `bulkPut()` with logic that checks for existing keys and compares values, throwing an error on mismatch. Test by simulating conflicting writes to ensure the old value is preserved.

索引モデルが issue の本文から書いたものです。