bench: add --mode=gen-osc to generate synthetic OSC sequences

This commit adds a few new mode flags to the `bench-stream` program
to generator synthetic OSC sequences. The new modes are `gen-osc`,
`gen-osc-valid`, and `gen-osc-invalid`. The `gen-osc` mode generates
equal parts valid and invalid OSC sequences, while the suffixed variants
are for generating only valid or invalid sequences, respectively.

This commit also fixes our build system to actually be able to build the
benchmarks. It turns out we were just rebuilding the main Ghostty binary
for `-Demit-bench`. And, our benchmarks didn't run under Zig 0.14, which
is now fixed.

An important new design I'm working towards in this commit is to split
out synthetic data generation to a dedicated package in
`src/bench/synth` although I'm tempted to move it to `src/synth` since
it may be useful outside of benchmarks.

The synth package is a work-in-progress, but it contains a hint of
what's to come. I ultimately want to able to generate all kinds of
synthetic data with a lot of knobs to control dimensionality (e.g. in
the case of OSC sequences: valid/invalid, length, operation types,
etc.).

src/bench/codepoint-width.zig

@@ -68,7 +68,7 @@ pub fn main() !void {
     var args: Args = .{};
     defer args.deinit();
     {
-        var iter = try std.process.argsWithAllocator(alloc);
+        var iter = try cli.args.argsIterator(alloc);
         defer iter.deinit();
         try cli.args.parse(Args, alloc, &args, &iter);
     }