Fuzz testing setup (cargo-fuzz)

[tec-eli]

Description

Set up cargo-fuzz and write fuzz targets to catch panics/overflows that structured tests
miss.

• Scope: fuzz/ (new directory at repo root)

Tasks

• Install/set up cargo-fuzz, create the fuzz/ directory
• Fuzz target: matrix construction from arbitrary bytes (dense, COO, CSR, CSC)
• Fuzz target: COO → CSR conversion
• Fuzz target: add_csr on two arbitrary CSR matrices
• Run each target for at least 30 minutes; add any findings as regression tests
• Confirm fuzz targets build with --no-default-features (no_std compatible)