[feature]: Add byte-based limit to universe proof cache

[ffranr]

The universe proof cache is currently limited by number of proofs per universe. Proof size varies and will grow over time so we need a total-byte-size cap.

Work:

• Add Size() uint64 to mssmt.Node, Leaf, and implementations.
• Add Size() uint64 to proof.Proof.
• Use these to track proof byte size in cache.
• Evict based on byte budget.
• Add unit tests for the size methods and eviction logic.

Output:

• Config option for max cache bytes.
• Byte-aware eviction in universe proof cache.

[ffranr]

We cannot track byte usage correctly unless the LRU reports whether Put replaced an existing entry or inserted a new one. That distinction matters because a replace may reduce or increase total bytes, while a new insert always increases it. The current API only exposes whether eviction occurred, so we need an additional signal from Put to detect replacement vs insertion. See current Put method:

// Put inserts a given (key,value) pair into the cache, if the key already
// exists, it will replace value and update it to be most recent item in cache.
// The return value indicates whether items had to be evicted to make room for
// the new element.
func (c *Cache[K, V]) Put(key K, value V) (bool, error) {

It probably makes sense just to add a byte size aware cache at this point.