Add a render-state row-cells getter that encodes the current cell's
full grapheme cluster directly as UTF-8 into a caller-provided
GhosttyBuffer. The getter writes the base codepoint first, followed by
any extra grapheme codepoints, and follows the existing buffer-writer
convention where len is bytes written on success or required capacity
on GHOSTTY_OUT_OF_SPACE.
Previously C consumers could query grapheme codepoints, but bindings
that needed UTF-8 text had to reconstruct and encode the cluster
themselves. That duplicated terminal internals in downstream bindings
and made users pay for awkward cross-language struct handling. By
owning the UTF-8/grapheme behavior in libghostty, bindings can use one
stable C API and optionally wrap it with small binding-local helpers.
Each C API file independently imported ../../lib/allocator.zig as
lib_alloc. Now that terminal/lib.zig re-exports the allocator module
as lib.alloc, use that instead. This removes the redundant import
and keeps all lib dependencies flowing through the single lib.zig
entry point.
Previously every file in the terminal package independently imported
build_options and ../lib/main.zig, then computed the same
lib_target constant. This was repetitive and meant each file needed
both imports just to get the target.
Introduce src/terminal/lib.zig which computes the target once and
re-exports the commonly used lib types (Enum, TaggedUnion, Struct,
String, checkGhosttyHEnum, structSizedFieldFits). All terminal
package files now import lib.zig and use lib.target instead of the
local lib_target constant, removing the per-file boilerplate.
