feat: add benchmarks, expanded proptest, and cargo-fuzz targets

[prestwich]

Summary

• Benchmarks: Add 5 new benchmark files — cursor writes, reserve vs put, nested transactions, concurrency (up to 128 readers), and scaling (100 to 100K entries × 32/128/512B values). Align all parity-tagged benchmarks with evmdb: 32-byte keys, 128-byte values, parameterized value sizes, standardized entry counts.
• Proptest: Split monolithic proptest_inputs.rs into 6 domain-focused files (kv, cursor, dupsort, dupfixed, iter, nested). Migrate all existing tests and add new cases for large values, multi-db isolation, DUPFIXED page spanning, nested txn semantics, and more.
• Fuzz: Set up cargo-fuzz with 6 targets focused on FFI/unsafe boundaries — Cow<[u8]> decode paths, fixed-size array decode, dirty page roundtrip, DUPFIXED page decode, and key validation.
• Input validation docs: Document the debug-only validation model prominently in README, lib.rs rustdoc, and CLAUDE.md. MDBX aborts the process on certain constraint violations; our debug assertions catch these during development, while release builds trust the caller.
• Cleanup: Remove dead return-borrowed feature documentation from iter module.

Test plan

• All unit tests pass (cargo t)
• All proptest files pass (65+ property tests)
• All benchmarks run clean (cargo bench)
• Clippy clean (--all-targets)
• cargo +nightly fmt applied
• Docker Linux tests pass
• Fuzz targets compile and run clean for 10s each (cargo +nightly fuzz run <target>)
• Doc tests pass

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