feat(vt-capi): port the libghostty-vt C API

Ports ZIG/src/lib_vt.zig and terminal/c/* as crates/ghostty-vt-capi
(cdylib + staticlib + rlib, soname libghostty-vt.so) implementing the
unchanged include/ghostty/vt.h + vt/*.h ABI: OSC parser, SGR parser,
key events and key encoding, paste safety, colors, result codes - all
38 native symbols, plus the ghostty_wasm_* helpers on wasm32.

Verification:
- tests/abi.rs generates a C probe from the headers, compiled with cc,
  that checks sizeof/alignof/offsetof of every struct/union, every
  enumerator and #define against Rust, and links every declared symbol;
  nm -D exports equal the declared functions.
- Ghostty's example/c-vt* programs compile unchanged against the Rust
  library (shared and static) and produce the expected output.
- The wasm32 build runs Ghostty's wasm-key-encode/wasm-sgr example
  scripts unmodified under node (upstream's wasm-sgr tag table is stale
  vs sgr.h; a regenerated copy checks full output).

Test Plan:
- cargo test -p ghostty-vt-capi (22 unit + 2 ABI + 5 C + 3 wasm tests)

crates/ghostty-vt-capi/examples/c/c-vt-key-encode.c

@@ -0,0 +1,59 @@
+#include <assert.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <string.h>
+#include <ghostty/vt.h>
+
+int main() {
+  GhosttyKeyEncoder encoder;
+  GhosttyResult result = ghostty_key_encoder_new(NULL, &encoder);
+  assert(result == GHOSTTY_SUCCESS);
+
+  // Set kitty flags with all features enabled
+  ghostty_key_encoder_setopt(encoder, GHOSTTY_KEY_ENCODER_OPT_KITTY_FLAGS, &(uint8_t){GHOSTTY_KITTY_KEY_ALL});
+
+  // Create key event
+  GhosttyKeyEvent event;
+  result = ghostty_key_event_new(NULL, &event);
+  assert(result == GHOSTTY_SUCCESS);
+  ghostty_key_event_set_action(event, GHOSTTY_KEY_ACTION_RELEASE);
+  ghostty_key_event_set_key(event, GHOSTTY_KEY_CONTROL_LEFT);
+  ghostty_key_event_set_mods(event, GHOSTTY_MODS_CTRL);
+  printf("Encoding event: left ctrl release with all Kitty flags enabled\n");
+
+  // Optionally, encode with null buffer to get required size. You can
+  // skip this step and provide a sufficiently large buffer directly.
+  // If there isn't enoug hspace, the function will return an out of memory
+  // error.
+  size_t required = 0;
+  result = ghostty_key_encoder_encode(encoder, event, NULL, 0, &required);
+  assert(result == GHOSTTY_OUT_OF_MEMORY);
+  printf("Required buffer size: %zu bytes\n", required);
+
+  // Encode the key event. We don't use our required size above because
+  // that was just an example; we know 128 bytes is enough.
+  char buf[128];
+  size_t written = 0;
+  result = ghostty_key_encoder_encode(encoder, event, buf, sizeof(buf), &written);
+  assert(result == GHOSTTY_SUCCESS);
+  printf("Encoded %zu bytes\n", written);
+
+  // Print the encoded sequence (hex and string)
+  printf("Hex: ");
+  for (size_t i = 0; i < written; i++) printf("%02x ", (unsigned char)buf[i]);
+  printf("\n");
+
+  printf("String: ");
+  for (size_t i = 0; i < written; i++) {
+    if (buf[i] == 0x1b) {
+      printf("\\x1b");
+    } else {
+      printf("%c", buf[i]);
+    }
+  }
+  printf("\n");
+
+  ghostty_key_event_free(event);
+  ghostty_key_encoder_free(encoder);
+  return 0;
+}