fix(os,termio): FreeBSD support, dev-profile resource precedence, docs

Final fix pass per the stopping criterion (docs/rewrite/PLAN.md),
resolving the remaining RV18 build/behaviour items (final-Z21.md):
- termio errno access uses the per-target libc accessor (__error on
  FreeBSD/DragonFly/Apple, __errno on OpenBSD/NetBSD/Android) instead of
  __errno_location everywhere, unblocking non-Linux BSD builds.
- FreeBSD resource discovery follows Zig (site-terminfo, local/share).
- Zig's Debug-only resource precedence (and the dev workspace-resources
  fallback) key off a ghostty_debug_build cfg set from PROFILE=="debug"
  instead of debug_assertions, so release builds with assertions honor
  GHOSTTY_RESOURCES_DIR first like Zig ReleaseSafe.
- KNOWN_DIFFERENCES.md lists the remaining P3 differences deliberately
  left unfixed (theme-preview editing details, a cleanup timeout edge
  case) and what cannot be verified on this machine.
- DEVIANT_FIXES.md registers the PTY-hangup output drain (35), the
  canvas BGRA conversion (36) and the dev resources fallback.

Test Plan:
- cargo test --workspace --no-fail-fast (exit 0)
- cargo check --target x86_64-unknown-freebsd -p ghostty-os

crates/ghostty-os/build.rs

@@ -0,0 +1,12 @@
+//! Emits `cfg(ghostty_debug_build)` for the Cargo `dev` profile (Zig
+//! `builtin.mode == .Debug`). `debug_assertions` is not equivalent: the
+//! repository's `release` profile enables it, like Zig's `ReleaseSafe`.
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-env-changed=PROFILE");
+    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");
+    }
+}

crates/ghostty-os/src/resourcesdir.rs

@@ -48,9 +48,13 @@ impl ResourcesDir {
 
 /// Sentinels we look for in the path to know we've found the resources
 /// directory (Linux).
-#[cfg(not(any(windows, target_os = "macos")))]
+#[cfg(not(any(windows, target_os = "macos", target_os = "freebsd")))]
 const SENTINELS: [&str; 2] = ["terminfo/g/ghostty", "terminfo/x/xterm-ghostty"];
 
+/// Sentinels on FreeBSD (`site-terminfo`, like Zig).
+#[cfg(target_os = "freebsd")]
+const SENTINELS: [&str; 2] = ["site-terminfo/g/ghostty", "site-terminfo/x/xterm-ghostty"];
+
 /// Sentinel on macOS (terminfo directories are named by hex there).
 #[cfg(target_os = "macos")]
 const SENTINELS: [&str; 1] = ["terminfo/78/xterm-ghostty"];
@@ -66,12 +70,14 @@ const DEV_SENTINELS: [&str; 1] = ["terminfo/x/xterm-ghostty"];
 /// Gets the directory to the bundled resources directory, if it
 /// exists (not all platforms or packages have it).
 ///
-/// Zig's `builtin.mode == .Debug` is mapped to `cfg!(debug_assertions)`.
+/// Zig's `builtin.mode == .Debug` is mapped to the Cargo `dev` profile
+/// (`cfg(ghostty_debug_build)`, set by `build.rs`), not to `debug_assertions`:
+/// the repository's `release` profile enables assertions like ReleaseSafe.
 pub fn resources_dir() -> io::Result<ResourcesDir> {
     let exe = std::env::current_exe().ok();
     let env = std::env::var_os("GHOSTTY_RESOURCES_DIR");
     Ok(resources_dir_impl(
-        cfg!(debug_assertions),
+        cfg!(ghostty_debug_build),
         exe.as_deref(),
         env,
     ))
@@ -112,9 +118,14 @@ fn resources_dir_impl(debug: bool, exe: Option<&Path>, env: Option<OsString>) ->
             }
         }
 
-        // We look for a /usr/share style path.
+        // We look for a /usr/share style path (/usr/local/share on FreeBSD).
+        let share = if cfg!(target_os = "freebsd") {
+            "local/share"
+        } else {
+            "share"
+        };
         for sentinel in SENTINELS {
-            if let Some(v) = maybe_dir_os(dir, "share", sentinel) {
+            if let Some(v) = maybe_dir_os(dir, share, sentinel) {
                 return ResourcesDir {
                     app_path: Some(enc(&v.join("ghostty"))),
                     host_path: None,

crates/ghostty-termio/src/command.rs

@@ -546,20 +546,38 @@ fn execvpe(prep: &Prepared, argv: &[*const c_char], envp: &[*const c_char]) -> E
     })
 }
 
-#[cfg(not(target_vendor = "apple"))]
+#[cfg(not(any(
+    target_vendor = "apple",
+    target_os = "freebsd",
+    target_os = "dragonfly",
+    target_os = "openbsd",
+    target_os = "netbsd",
+    target_os = "android"
+)))]
 #[allow(unsafe_code)]
 fn errno_now() -> i32 {
     // SAFETY: __errno_location returns a valid thread-local pointer.
     unsafe { *libc::__errno_location() }
 }
 
-#[cfg(target_vendor = "apple")]
+#[cfg(any(
+    target_vendor = "apple",
+    target_os = "freebsd",
+    target_os = "dragonfly"
+))]
 #[allow(unsafe_code)]
 fn errno_now() -> i32 {
     // SAFETY: __error returns a valid thread-local pointer.
     unsafe { *libc::__error() }
 }
 
+#[cfg(any(target_os = "openbsd", target_os = "netbsd", target_os = "android"))]
+#[allow(unsafe_code)]
+fn errno_now() -> i32 {
+    // SAFETY: __errno returns a valid thread-local pointer.
+    unsafe { *libc::__errno() }
+}
+
 fn write_stderr(msg: &[u8]) {
     // Best effort, no allocation.
     let _ = rustix::io::write(borrow_stderr(), msg);