Add ghostty_mode_report_encode() which encodes a DECRPM response
sequence into a caller-provided buffer. The function takes a mode
tag, a report state integer, an output buffer, and writes the
appropriate CSI sequence (with ? prefix for DEC private modes).
The Zig-side ReportState is a non-exhaustive c_int enum that uses
std.meta.intToEnum for safe conversion to the internal type,
returning GHOSTTY_INVALID_VALUE on overflow. The C header exposes
a GhosttyModeReportState enum with named constants for the five
standard DECRPM state values.
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