feat: implement Merkle Tree proof verification (issue #332)

[spiffamani]

Closes #332

Summary

Implements Merkle Tree proof generation and verification for SoroScope state commitments. The existing merkle_tree.rs had broken code (orphaned blocks outside functions, duplicate function bodies, no proof logic) — this PR fixes the entire file and adds the requested verify_proof() function.

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Changes

core/src/merkle_tree.rs

• Fixed broken file — removed orphaned code blocks and duplicate function bodies
• Added ProofNode struct — holds a sibling hash and which side the path node sits on at each level
• Added MerkleProof struct — holds the leaf, the full proof path, and the root it was generated against
• Rewrote build() — now stores all tree levels internally so proofs can be generated after building
• Added generate_proof(leaf_index) — walks from the leaf up to the root collecting sibling hashes at each level to produce a complete inclusion proof
• Added verify_proof(proof, root) — the core requirement from issue Core: Implement Merkle Tree proof verification #332; recomputes the root by combining hashes level by level using the proof nodes and returns true only if the computed root matches the supplied root
• Hashing convention: pairs are sorted before combining (min || max), which is the OpenZeppelin/standard approach and makes proofs order-independent while preventing second-preimage attacks
• Odd nodes at any level are promoted by hashing with themselves hash(x || x)

core/src/lib.rs

• Exposed merkle_tree as a public module

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Tests (20 total)

Build tests

• test_build_single_leaf — root equals the hash of the single leaf
• test_build_two_leaves — root is non-zero
• test_build_even_leaves — 4 leaves builds correctly
• test_build_odd_leaves — 3 leaves builds correctly (odd promotion)
• test_build_empty_returns_error — empty input returns error
• test_same_leaves_same_root — deterministic root
• test_different_leaves_different_root — different data produces different root
• test_get_root_hex_length — hex string is 64 characters

generate_proof tests

• test_generate_proof_before_build_returns_error
• test_generate_proof_out_of_range_returns_error
• test_generate_proof_single_leaf_has_no_nodes
• test_generate_proof_two_leaves_has_one_node

verify_proof tests

• test_verify_proof_valid_two_leaves — valid proof verifies correctly
• test_verify_proof_valid_right_leaf — right-side leaf verifies correctly
• test_verify_proof_valid_four_leaves_all_indices — all 4 indices valid
• test_verify_proof_valid_odd_leaf_count — all 3 indices valid with odd tree
• test_verify_proof_valid_single_leaf — single leaf proof verifies
• test_verify_proof_valid_large_tree — all 8 indices valid in an 8-leaf tree
• test_verify_proof_tampered_leaf_fails — swapped leaf returns false
• test_verify_proof_tampered_sibling_fails — corrupted sibling returns false
• test_verify_proof_wrong_root_fails — wrong root returns false
• test_verify_proof_cross_tree_fails — proof from tree A does not verify against tree B root

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