ci: enforce Clippy warnings for the contract library

[mikewheeleer]

Summary

• add a strict cargo clippy --lib -- -D warnings gate to the shared Makefile and CI workflow
• remove type-preserving pagination casts and restore the typed storage accessors that were preventing the library from compiling
• preserve the established Soroban event wire format with a narrowly scoped deprecation allowance
• document the pre-fix inventory, scope decision, allow policy, and wasm verification in docs/CLIPPY-AUDIT.md

Audit result

The pinned stable toolchain reported 38 production findings before fixes: 27 deprecated Events::publish calls, 10 unnecessary u32 casts, and one unused typed storage probe. The 10 casts were removed. The event migration is deferred because switching to #[contractevent] can change the compatibility-sensitive serialized event shape. The two intentionally retained dead-code probes have narrow, documented allowances; no crate-level blanket allowance was added.

Test-target linting is explicitly deferred: the checked-in event regression block does not compile against Soroban SDK 25.3 (Address::random and ContractEvents::len/get are unavailable), so cargo test and cargo clippy --all-targets cannot reach a test lint inventory. This PR does not suppress those failures or change test behavior.

Verification

• cargo fmt --all -- --check
• make clippy
• cargo build
• cargo build --target wasm32-unknown-unknown --release (92 KiB artifact)
• cargo test --lib --no-run remains blocked by the pre-existing test/API incompatibility described above

The release wasm artifact builds successfully; a before/after byte delta is not available because the upstream baseline source does not compile before this prerequisite repair.

Closes #261