fix: ignore spurious theme watcher reloads

[subinium]

Summary

• compare theme source contents before surfacing a hot reload
• keep parent-directory watches portable across macOS FSEvents, Windows, and Linux while ignoring registration, duplicate, and sibling-file events
• isolate watcher tests in a unique temporary directory and cover same-content and sibling writes

Validation

• cargo fmt -- --check
• cargo check --all-features
• cargo clippy --all-features -- -D warnings
• cargo test --all-features
• cargo check --examples --all-features
• typos
• cargo check -p superlighttui --no-default-features
• cargo check -p slt-wasm --target wasm32-unknown-unknown
• cargo hack check -p superlighttui --each-feature --no-dev-deps
• cargo audit
• cargo deny check
• RUSTFLAGS="-Wmissing_docs" cargo check --all-features
• theme watcher regression test repeated 30 times on macOS