fix(build): avoid perpetually invalidating build script inputs

Repeated production builds recompiled build-info, GTK and their dependents
because build scripts watched missing optional files and generated outputs.
Watch packed-refs only when present and the prebuilt GTK bundle only when
consumed. Loose refs remain watched so packing them refreshes Git inputs.

Invoke glib-compile-resources directly instead of using a helper that also
watches generated XML and UI files. The existing resources directory watch
covers their source inputs. Remove the unused helper dependency and document
the build input rules.

Test Plan:
- Run cargo build --profile production three times; repeats compile nothing.
- Run cargo clippy, cargo clippy --benches and cargo clippy --tests.
- Run cargo +nightly fmt and repeat all three Clippy checks.

Documentation: docs/rewrite/ARCHITECTURE.md

docs/rewrite/ARCHITECTURE.md

@@ -70,6 +70,16 @@ ghostty-bench (benchmarks), ghostty-build-data (completions, man pages, syntax).
 Non-code: `resources/` (shell-integration, themes, terminfo), `dist/` (desktop
 integration), `po/` (translations), crate-local `res/`/`shaders/`/`resources/`.
 
+Build inputs: `ghostty-build-info/build.rs` derives version metadata from the
+workspace manifest and Git HEAD/refs, watching `packed-refs` only when it exists
+(packing loose refs also changes the watched `refs` directory). The GTK build
+script compiles `resources/` in checkouts and watches the optional
+`dist/ghostty.gresource` bundle only when consuming it from a source tarball.
+Missing optional files must not be emitted as Cargo `rerun-if-changed` inputs:
+they invalidate every subsequent build and rebuild downstream crates.
+GTK invokes `glib-compile-resources` directly so generated XML/UI files in
+`OUT_DIR` are not watched as inputs; the `resources/` watch covers their sources.
+
 ## Key design decisions
 
 1. **Safe Rust by default.** Workspace lint `unsafe_code = "forbid"`. Crates