fix: close remaining gaps found by the completeness audit

Addresses the partial ports listed in
docs/rewrite/notes/review-completeness-codex.md (details in
docs/rewrite/notes/gaps-X2.md):
- Flatpak: the exit watcher no longer consumes the host command; a
  shared FlatpakSignalHandle lets closing a surface still SIGHUP the
  host process group.
- Child environment is byte-preserving end to end (ghostty_os::EnvMap
  of OsStrings) and Snap paths are filtered like filterSnapPaths.
- argv handling no longer panics on non-UTF-8 arguments.
- URI percent-decoding keeps raw bytes; OSC 7 pwd is passed through
  without lossy conversion.
- Keybinding comparison/hashing uses full Unicode case folding from a
  table generated from CaseFolding.txt (Unicode 17.0.0).
- Link regexes set Oniguruma's whole-search retry limit through the new
  minimal ghostty-onig crate (one SAFETY-commented FFI call).
- OSC 9 NUL-capacity check ordering matches Zig.
- Wayland registry binds/clears globals advertised after startup.
- Custom shaders may pass sampler2D to helper functions (naga adapter).
- +list-themes preview renders OSC 8 links, styled underlines and
  follows color-scheme reports.

Also includes the inspector's integration edits in shared files
(renderer state hook, termio VT recording, core/apprt-gtk wiring), X1's
config/input/cli additions for completion generation, and a fix for a
race in the termio backpressure test (it wrote before `stty raw`
applied, so the cooked line discipline dropped bytes past MAX_CANON).

Test Plan:
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings

crates/ghostty-core/tests/common/mod.rs

@@ -143,12 +143,12 @@ impl RtSurface for FakeRt {
             }
         }
     }
-    fn default_termio_env(&self) -> Vec<(String, String)> {
+    fn default_termio_env(&self) -> Vec<(std::ffi::OsString, std::ffi::OsString)> {
         // Like GTK: the process environment minus what the runtime does not
         // want to leak (here HOME stands in for GDK_DISABLE & co), plus extras.
-        let mut env: Vec<(String, String)> =
-            std::env::vars().filter(|(k, _)| k != "HOME").collect();
-        env.push(("TERM_PROGRAM_TEST".to_owned(), "1".to_owned()));
+        let mut env: Vec<(std::ffi::OsString, std::ffi::OsString)> =
+            std::env::vars_os().filter(|(k, _)| k != "HOME").collect();
+        env.push(("TERM_PROGRAM_TEST".into(), "1".into()));
         env
     }
     fn gl_proc_address(&self, name: &str) -> *const std::ffi::c_void {