input: extract mouse encoding to a pure, testable file

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.

src/input/mouse.zig

@@ -1,5 +1,11 @@
 const std = @import("std");
 
+/// The type of action associated with a mouse event. This is different
+/// from ButtonState because button state is simply the current state
+/// of a mouse button but an action is something that triggers via
+/// an GUI event and supports more.
+pub const Action = enum { press, release, motion };
+
 /// The state of a mouse button.
 ///
 /// This is backed by a c_int so we can use this as-is for our embedding API.