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/result.h

@@ -0,0 +1,22 @@
+/**
+ * @file result.h
+ *
+ * Result codes for libghostty-vt operations.
+ */
+
+#ifndef GHOSTTY_VT_RESULT_H
+#define GHOSTTY_VT_RESULT_H
+
+/**
+ * Result codes for libghostty-vt operations.
+ */
+typedef enum {
+    /** Operation completed successfully */
+    GHOSTTY_SUCCESS = 0,
+    /** Operation failed due to failed allocation */
+    GHOSTTY_OUT_OF_MEMORY = -1,
+    /** Operation failed due to invalid value */
+    GHOSTTY_INVALID_VALUE = -2,
+} GhosttyResult;
+
+#endif /* GHOSTTY_VT_RESULT_H */