fix(imt): use index-based parent check for lastSubtrees in _update

[0x471]

Description

This PR fixes _update in both InternalBinaryIMT and InternalQuinaryIMT by replacing the value-based equality checks on lastSubtrees with deterministic index-based parent comparison.

Now we only update lastSubtrees[i] when the update and last-insert paths share the same parent node at level i+1. Also moved the bounds check before the main loop to prevent a uint256 underflow panic on empty trees

Related Issue(s)

Closes #41

Checklist

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