v2: support querying and restoring cursor style via DECRQSS

Currently, Bubble Tea always restores the cursor to the default blinking block cursor style. Instead, and if the terminal supports DECRQSS, we should query for the existing style before changing it so that we can restore the same style later.

Start by tracing Bubble Tea’s cursor-style restoration path and the terminal escape-sequence handling used when the program exits. Read the DECRQSS terminal protocol and existing cursor-style behavior, then verify that supported terminals restore the queried style while unsupported terminals retain the current behavior.

Written by the indexing model from the issue text.