lib-vt: C API for SGR parser (#9352)

This exposes the SGR parser to the C and Wasm APIs. An example is shown
in c-vt-sgr.

Compressed example:

```c
#include <assert.h>
#include <stdio.h>
#include <ghostty/vt.h>

int main() {
  // Create parser
  GhosttySgrParser parser;
  assert(ghostty_sgr_new(NULL, &parser) == GHOSTTY_SUCCESS);

  // Parse: ESC[1;31m (bold + red foreground)
  uint16_t params[] = {1, 31};
  assert(ghostty_sgr_set_params(parser, params, NULL, 2) == GHOSTTY_SUCCESS);

  printf("Parsing: ESC[1;31m\n\n");

  // Iterate through attributes
  GhosttySgrAttribute attr;
  while (ghostty_sgr_next(parser, &attr)) {
    switch (attr.tag) {
      case GHOSTTY_SGR_ATTR_BOLD:
        printf("✓ Bold enabled\n");
        break;
      case GHOSTTY_SGR_ATTR_FG_8:
        printf("✓ Foreground color: %d (red)\n", attr.value.fg_8);
        break;
      default:
        break;
    }
  }

  ghostty_sgr_free(parser);
  return 0;
}
```

**AI disclosure:** Amp wrote most of the C headers, but I verified it
all. https://ampcode.com/threads/T-d9f145cb-e6ef-48a8-ad63-e5fc85c0d43e

.github/workflows/test.yml

@@ -94,7 +94,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        dir: [c-vt, c-vt-key-encode, c-vt-paste, zig-vt, zig-vt-stream]
+        dir:
+          [c-vt, c-vt-key-encode, c-vt-paste, c-vt-sgr, zig-vt, zig-vt-stream]
     name: Example ${{ matrix.dir }}
     runs-on: namespace-profile-ghostty-sm
     needs: test