perf(store): avoid duplicate nullifier iteration

[Mr-kay-cloud2]

Summary

• Avoids traversing block-created nullifiers twice in State::compute_tree_mutations.
• Builds the nullifier mutation input while checking for duplicate nullifiers.
• Reuses the prepared mutations when computing the nullifier tree update.

Closes #1548

Testing

• rustfmt +nightly --edition 2024 crates/store/src/state/apply_block.rs
• cargo check -p miden-node-store --all-targets could not run locally on Windows because the miden-agglayer dependency checkout fails on an invalid : path in a submodule file.