Implement rehashing for dirty BMTs in StateValue

[ysh-red]

Trigger: Any export circuit that performs MerkleTree.checkRoot() (a read) followed by MerkleTree.insert() (a write) — even on the same tree, even in the same queryLedgerState call.
Panic message:

Error: invalid operation for type: attempted to take root of non-rehashed bmt (this should not be possible!)
  at partitionTranscripts (...)
  at partitionTranscript (.../compact-js/.../ContractExecutable.js)

Minimal Reproducing example :

export ledger tree: MerkleTree<10, Bytes<32>>;

witness get_path(cm: Bytes<32>): Maybe<MerkleTreePath<10, Bytes<32>>>;
witness get_a(): Bytes<32>;
witness get_b(): Bytes<32>;

export circuit transfer(): [] {
  const cm = get_a();
  const p = get_path(cm);
  // READ: checkRoot — passes fine in isolation
  assert(tree.checkRoot(disclose(merkleTreePathRoot<10, Bytes<32>>(p.value))), "bad root");
  // WRITE: insert — causes partitionTranscripts to panic
  tree.insert(disclose(get_a()));
  tree.insert(disclose(get_b()));
}

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