fuzz: add OSC parser fuzzer

test/fuzz-libghostty/src/fuzz_osc.zig

@@ -0,0 +1,45 @@
+const std = @import("std");
+const ghostty_vt = @import("ghostty-vt");
+const mem = @import("mem.zig");
+const osc = ghostty_vt.osc;
+
+/// Use a single global allocator for simplicity and to avoid heap
+/// allocation overhead in the fuzzer. The allocator is backed by a fixed
+/// buffer, and every fuzz input resets the bump pointer to the start.
+var fuzz_alloc: mem.FuzzAllocator(8 * 1024 * 1024) = .{};
+
+pub export fn zig_fuzz_init() callconv(.c) void {
+    fuzz_alloc.init();
+}
+
+pub export fn zig_fuzz_test(
+    buf: [*]const u8,
+    len: usize,
+) callconv(.c) void {
+    // Need at least one byte for the terminator selector.
+    if (len == 0) return;
+
+    fuzz_alloc.reset();
+    const alloc = fuzz_alloc.allocator();
+    const input = buf[0..len];
+
+    // Use the first byte to select the terminator variant.
+    const selector = input[0];
+    const payload = input[1..];
+
+    var p = osc.Parser.init(alloc);
+    defer p.deinit();
+    for (payload) |byte| p.next(byte);
+
+    // Exercise all three terminator paths:
+    //   0 -> BEL  (0x07)
+    //   1 -> ST   (0x9c)
+    //   2 -> missing terminator (null)
+    const terminator: ?u8 = switch (selector % 3) {
+        0 => 0x07,
+        1 => 0x9c,
+        else => null,
+    };
+
+    _ = p.end(terminator);
+}