feat: SmtStorageReader trait

[sergerad]

Description

Extract a new SmtStorageReader trait from SmtStorage, enabling LargeSmt<S> to work with read-only storage for read operations, and only require SmtStorage for mutations.

Motivation

Previously, LargeSmt<S: SmtStorage> required full read+write storage for all operations, including purely read-only ones like get_leaf, get_value, open, and tree loading. This made it impossible to use LargeSmt with a read-only storage backend.

In miden-node (PR), we are working towards removing synchronization mechanisms by providing read-only types which can be cloned.

Changes

Trait hierarchy (storage/mod.rs):

• SmtStorageReader — &self methods (leaf/subtree reads, iterators, get_depth24)
• SmtStorage: SmtStorageReader — &mut self methods (inserts, removes, apply)

LargeSmt<S> struct bound relaxed (large/mod.rs):

• Struct bound changed from S: SmtStorage to S: SmtStorageReader
• Impl blocks split by required capability:
  • impl<S: SmtStorageReader> — accessors (root, get_leaf, get_value, open, is_empty), iterators, construction from existing storage (new, load, load_with_root)
  • impl<S: SmtStorage> — insert, with_entries, batch construction methods

Other files updated:

• smt_trait.rs — SparseMerkleTree impl bounded by SmtStorage
• batch_ops.rs — bounded by SmtStorage
• construction.rs — split into Reader (load from storage) and Storage (build with entries) blocks
• iter.rs — bounded by SmtStorageReader
• memory.rs, rocksdb.rs — split impl SmtStorage into impl SmtStorageReader + impl SmtStorage
• smt/mod.rs — re-exports SmtStorageReader

Backwards compatibility

SmtStorage retains all original methods (read + write) via its supertrait SmtStorageReader, so existing code using S: SmtStorage continues to compile unchanged.

[iamrecursion]

I have not read the code as it is still a draft, but please make sure that this does not incur any performance regressions for LargeSmt.

[sergerad]

Closing in favour of #967 which points at main for v0.23.1