Use configured array length for usize literal generation

[uurl]

Closes #369

Summary

Use TyConfig::array_max_len when generating small usize literals instead of relying on the hard-coded value 8.

Changes

• Split the existing Sombrero distribution into usize-specific and isize-specific distributions.
• Parameterize the usize distribution with the configured upper bound.
• Pass tcx.config.array_max_len from GenLiteral::gen_literal.
• Remove the FIXME associated with the hard-coded array-length value.

Rationale

Array lengths are configurable, so duplicating the default value in the literal generator can produce inconsistent behavior when array-max-len is changed.

Using the value from TyCtxt keeps literal generation aligned with type generation.

Validation

• cargo test
• cargo clippy --all-targets
• cargo fmt --all --check
• cd crates/fuzzer/rustlantis && cargo test -p generate usize_sombrero_uses_configured_small_values_upper_bound
• cd crates/fuzzer/rustlantis && cargo test -p generate
• cd crates/fuzzer/rustlantis && cargo clippy -p generate --all-targets
• cd crates/fuzzer/rustlantis && cargo fmt --all -- --check