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-vt/src/osc/mod.rs

@@ -442,6 +442,17 @@ impl Parser {
         Some(self.buffer.clone())
     }
 
+    /// Zig `writer.writeByte(0)` capacity check: when the fixed buffer is
+    /// full there is no room for the NUL terminator, so the parser becomes
+    /// invalid and this returns false.
+    pub fn reserve_nul(&mut self) -> bool {
+        if self.writer == Writer::Fixed && self.buffer.len() >= Self::MAX_BUF {
+            self.state = State::Invalid;
+            return false;
+        }
+        true
+    }
+
     /// Like [`Parser::data`] but without reserving room for the NUL
     /// terminator (for parsers that, like Zig's color parsers, read
     /// `writer.buffered()` directly).