Motivation

• Tighten property-test coverage around seed parsing to ensure ambiguous 64-byte inputs are handled strictly.
• Make cache semantics explicit in tests by verifying pointer identity behavior across cache clears while preserving deterministic values.

Description

• Add a proptest seed_from_env_value_invalid_64_byte_hex_is_err that generates 64-char non-hex payloads and asserts Seed::from_env_value returns Err for raw, 0x-prefixed, and 0X-prefixed forms.
• Extend prop_cache_hit_returns_same_arc to call fx.clear_cache() and assert the cached Arc pointer is replaced while the dereferenced deterministic value remains equal.
• Tests live in crates/uselesskey-core/tests/core_prop.rs and use existing proptest helpers and generators.

Testing

• Ran cargo test -p uselesskey-core --test core_prop, which executed the updated proptests.
• All tests in that test target passed (17 passed, 0 failed).

Codex Task