Simplify the update() method

Improves readability and removes an unnecessary allocation in the update() method.

Changes:

• Replaced tuple-based side-effect expressions (let (..) = (...)) with direct method calls (self.render.clear(), etc.).
• Removed c.into() and used push() for non-tab characters, avoiding the creation of a temporary String.
• Replaced (rx, cx) = (...) with rx += ...; cx += ...;, which is more idiomatic and easier to read.

No functional changes intended.