Previously, every call to vt_write created a fresh ReadonlyStream with
new Parser and UTF8Decoder state. This meant escape sequences split
across write boundaries (e.g. ESC in one write, [27m in the next)
would lose parser state, causing the second write to start in ground
state and print the CSI parameters as literal text.
The C API now stores a persistent ReadonlyStream in the TerminalWrapper
struct, which is created when the Terminal is initialized. The vt_write
function feeds bytes through this stored stream, allowing it to maintain
parser state across calls. This change ensures that escape sequences
split across write boundaries are correctly parsed and rendered.
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