Expose the internal mouse encoding functionality through the C API,
following the same pattern as the existing key encoding API. This
allows external consumers of libvt to encode mouse events into
terminal escape sequences (X10, UTF-8, SGR, URxvt, SGR-Pixels).
The API is split into two opaque handle types: GhosttyMouseEvent
for building normalized mouse events (action, button, modifiers,
position) and GhosttyMouseEncoder for converting those events into
escape sequences. The encoder is configured via a setopt interface
supporting tracking mode, output format, renderer geometry, button
state, and optional motion deduplication by last cell.
Encoder state can also be bulk-configured from a terminal handle
via ghostty_mouse_encoder_setopt_from_terminal. Failed encodes due
to insufficient buffer space report the required size without
mutating deduplication state.
Move mouse event encoding logic from Surface.zig into a new
input/mouse_encode.zig file.
The new file encapsulates event filtering (shouldReport),
button code computation, viewport bounds checking, motion
deduplication, and all five wire formats (X10, UTF-8, SGR,
urxvt, SGR-pixels). This makes the encoding independently
testable and adds unit tests covering each format and edge
case.
Additionally, Surface `mouseReport` can no longer fail, since the only
failure mode is no buffer space which should be impossible. Updated
the signature to remove the error set.
