fix(renderer): propagate style to generated operators

[floze-the-genius]

Closes #24

Summary

• apply the active render style to negation, prime, and infix layout nodes
• ensure generated -, ', and binary operator cells use the same color and attributes as their surrounding expression
• add backend-level regression coverage for standalone operators and the nested colored integral from the issue

Why

Rendered child nodes already carried RenderCtx::current_style, but punctuation generated directly by these composite layout nodes still used the default empty style. That left operators uncolored inside styled expressions even though adjacent identifiers were styled correctly.

Tests

• cargo fmt --check
• cargo test
• cargo clippy -- -D warnings

cargo test --all-features is currently blocked on main by the pre-existing unresolved crate::backends::memory import in src/ratatui.rs; the repository's configured CI commands above all pass.

[thatmagicalcat]

LGTM