## Summary
Mirror the `libghostty-vt-static` pkg-config pattern from #12210 for the
internal library.
- Add `ghostty-internal.pc` (shared, `-lghostty`) and
`ghostty-internal-static.pc` (static, direct archive reference) so
consumers can discover either variant via pkg-config
- Named `ghostty-internal` to distinguish from the public
`libghostty-vt` API
- Static module points at the platform-correct archive name
(`ghostty-static.lib` on Windows, `libghostty.a` elsewhere)
- pkg-config files are generated during shared builds and installed via
`GhosttyLib.install()`
## Test plan
- [x] `zig build` succeeds (default target)
- [x] `ghostty-internal.pc` and `ghostty-internal-static.pc` appear in
`zig-out/share/pkgconfig/`
- [x] Static `.pc` points at `ghostty-static.lib` (Windows) /
`libghostty.a` (Unix)
- [x] Shared `.pc` uses standard `-L -l` flags
- [x] Existing `libghostty-vt` pkg-config files are unaffected
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