fix(store): replace panics with error returns in SMT

[yasinlex]

Problem

The SMT (Sparse Merkle Tree) implementation in store/smt.go uses panic() in 5 places for conditions that should return errors. In a production blockchain node, these panics cause an unrecoverable crash when encountering corrupted data or database errors.

Changes

getOtherChild() (line 1156)

• Before: panics if child key doesn't match
• After: returns ErrChildNotFound error

replaceChild() (line 1167)

• Before: panics if old key doesn't match any child
• After: returns ErrChildNotReplaced error

initializeTree() (line 481)

• Before: 4 panic() calls on database write failures
• After: returns errors from each database operation

NewSMT() (line 154)

• Before: panics on getNode() error and calls initializeTree() without error handling
• After: returns errors from both operations

NewDefaultSMT() (line 149)

• Before: no error return
• After: propagates error from NewSMT()

New Error Types

Added to store/error.go:

• ErrChildNotFound(childKey string) - for missing child nodes
• ErrChildNotReplaced(oldKey string) - for failed child replacements

Impact

• Before: Corrupted SMT data on disk → node crashes with panic
• After: Corrupted SMT data on disk → error propagated to caller, node can handle gracefully

Testing

• All existing tests pass
• Build succeeds with go build ./store/...