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/include/ghostty/vt/key.h

@@ -0,0 +1,80 @@
+/**
+ * @file key.h
+ *
+ * Key encoding module - encode key events into terminal escape sequences.
+ */
+
+#ifndef GHOSTTY_VT_KEY_H
+#define GHOSTTY_VT_KEY_H
+
+/** @defgroup key Key Encoding
+ *
+ * Utilities for encoding key events into terminal escape sequences,
+ * supporting both legacy encoding as well as Kitty Keyboard Protocol.
+ *
+ * ## Basic Usage
+ *
+ * 1. Create an encoder instance with ghostty_key_encoder_new()
+ * 2. Configure encoder options with ghostty_key_encoder_setopt().
+ * 3. For each key event:
+ *    - Create a key event with ghostty_key_event_new()
+ *    - Set event properties (action, key, modifiers, etc.)
+ *    - Encode with ghostty_key_encoder_encode()
+ *    - Free the event with ghostty_key_event_free()
+ *    - Note: You can also reuse the same key event multiple times by
+ *      changing its properties.
+ * 4. Free the encoder with ghostty_key_encoder_free() when done
+ *
+ * ## Example
+ *
+ * @code{.c}
+ * #include <assert.h>
+ * #include <stdio.h>
+ * #include <ghostty/vt.h>
+ * 
+ * int main() {
+ *   // Create encoder
+ *   GhosttyKeyEncoder encoder;
+ *   GhosttyResult result = ghostty_key_encoder_new(NULL, &encoder);
+ *   assert(result == GHOSTTY_SUCCESS);
+ * 
+ *   // Enable Kitty keyboard protocol with all features
+ *   ghostty_key_encoder_setopt(encoder, GHOSTTY_KEY_ENCODER_OPT_KITTY_FLAGS, 
+ *                              &(uint8_t){GHOSTTY_KITTY_KEY_ALL});
+ * 
+ *   // Create and configure key event for Ctrl+C press
+ *   GhosttyKeyEvent event;
+ *   result = ghostty_key_event_new(NULL, &event);
+ *   assert(result == GHOSTTY_SUCCESS);
+ *   ghostty_key_event_set_action(event, GHOSTTY_KEY_ACTION_PRESS);
+ *   ghostty_key_event_set_key(event, GHOSTTY_KEY_C);
+ *   ghostty_key_event_set_mods(event, GHOSTTY_MODS_CTRL);
+ * 
+ *   // Encode the key event
+ *   char buf[128];
+ *   size_t written = 0;
+ *   result = ghostty_key_encoder_encode(encoder, event, buf, sizeof(buf), &written);
+ *   assert(result == GHOSTTY_SUCCESS);
+ * 
+ *   // Use the encoded sequence (e.g., write to terminal)
+ *   fwrite(buf, 1, written, stdout);
+ * 
+ *   // Cleanup
+ *   ghostty_key_event_free(event);
+ *   ghostty_key_encoder_free(encoder);
+ *   return 0;
+ * }
+ * @endcode
+ *
+ * For a complete working example, see example/c-vt-key-encode in the
+ * repository.
+ *
+ * @{
+ */
+
+#include <ghostty/vt/key/event.h>
+#include <ghostty/vt/key/encoder.h>
+
+/** @} */
+
+#endif /* GHOSTTY_VT_KEY_H */