refactor(libghostty): restructure and harden native and Wasm C ABI bindings

[elias8]

Redesign libghostty’s C ABI seam around focused native and Wasm bindings while preserving existing behavior and keeping platform details internal. Strengthen runtime layout, result, ownership, lifetime, callback, and memory safety; improve hot path allocation behavior; expose idiomatic public wrappers; and align ABI and behavioral tests with the shared native and Wasm contracts.

This change touches nearly every file in the libghostty package and is close to a complete internal rewrite, while preserving existing behavior and most of the public API. The public API was already in a good place, but the internals had not kept pace with the growth of the C ABI since the package’s initial implementation. I have wanted to do this refactoring for some time. The resulting structure gives each platform binding a clearer boundary, makes behavioral consistency between native and Wasm explicit and enforceable, and should make future ABI additions easier to implement and maintain.

I plan a few focused follow up PRs, mainly around the build system and further quality improvements.

Note: I planned and iterated on these changes myself, while using LLMs extensively for the mechanical implementation work.