feat-deviant(os): embed a configurable resource directory

Relocating a Cargo-installed binary loses executable-relative resource
lookup even when themes and shell integration exist under /usr/share.
Allow packagers to set GHOSTTY_BUILD_RESOURCES_DIR when compiling.

Absolute paths are used directly; relative paths resolve against the
running executable's directory for relocatable installations. Explicit
paths do not require terminfo sentinels. A nonempty runtime override wins
in every profile when the build option is configured. Unset or empty
build options preserve existing discovery, including debug precedence.
Cargo watches the build option so changing or unsetting it recompiles
lookup. Resource files still need to be installed separately.

Document this requested upstream deviation and the build option in the
README, architecture overview, and deviation register.

Test Plan:
- Eight resource lookup unit tests pass.
- Nightly formatting and ghostty-os Clippy with -D warnings pass.
- Production build with /usr/share/ghostty succeeds; a copied executable
  lists all 463 themes with no runtime override.
- Compiled probes verify absolute, relative, unset, and empty options,
  Cargo rebuilds, relocation, runtime overrides, and unrelated cwd.

Deviation-Register: docs/rewrite/DEVIANT_FIXES.md

crates/ghostty-os/build.rs

@@ -5,6 +5,8 @@
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rerun-if-env-changed=PROFILE");
+    // Recompile resource lookup when the packager changes the embedded path.
+    println!("cargo:rerun-if-env-changed=GHOSTTY_BUILD_RESOURCES_DIR");
     println!("cargo:rustc-check-cfg=cfg(ghostty_debug_build)");
     if std::env::var("PROFILE").is_ok_and(|p| p == "debug") {
         println!("cargo:rustc-cfg=ghostty_debug_build");