Introduce atomic runtime option updates: updateOptions()

[nickesc]

Adds a controlled, atomic way to update terminal configuration at runtime; breaking change that prevents consumers from mutating options individually or directly.

• Replaces the mutable TermOptions class and TermOptionsConfig interface with a readonly TermOptions interface
• Adds Terminal.updateOptions(partialOptions) method, which merges updates into a frozen options snapshot and replaces it atomically
• Ensures prompt and preprompt changes preserve unfinished input for initialized terminals and cause a redraw with the new prompt

closes #4