chore: pre-release review cleanup items

[rocketman-code]

Current Behavior

Three minor code quality issues identified during v0.4.0 pre-release review:

1. tempfile is listed in [dependencies] but only used in #[cfg(test)] modules
2. Provably-safe .unwrap() calls lack expect("reason") documentation of their invariants
3. notify crate only enables macos_fsevent feature -- file watching silently degrades to no-op on Linux/Windows

Expected Behavior

1. tempfile should be in [dev-dependencies] to avoid shipping it in production builds
2. Invariant-dependent .unwrap() calls should use expect() so violated assumptions produce clear messages
3. Cross-platform file watching behavior should be documented or addressed with platform-specific features

Context

Identified during v0.4.0 pre-release code review. None are correctness issues -- the code handles all three gracefully. These are maintainability and packaging improvements.

Technical Details

Relevant Code

Cargo.toml:35

tempfile = "3"    # only used in #[cfg(test)] blocks

Cargo.toml:38

notify = { version = "7", default-features = false, features = ["macos_fsevent"] }

Bare .unwrap() instances in production code: cache.rs:193-198, session.rs:204,236,451, walker.rs:122,142, query.rs:571, main.rs:484, repl.rs:692,730