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-input/src/binding/action.rs

@@ -1225,7 +1225,17 @@ impl ActionKey {
         }
     }
 
-    /// The doc comment of the action (Zig `help_strings.KeybindAction`).
+    /// The Zig `help_strings.KeybindAction` string: [`ActionKey::docs`]
+    /// without the single trailing newline every doc literal carries. (A
+    /// trailing empty `///` line in the Zig source is a second `\n`.)
+    #[must_use]
+    pub fn help_string(self) -> &'static str {
+        let d = self.docs();
+        d.strip_suffix('\n').unwrap_or(d)
+    }
+
+    /// The doc comment of the action (Zig `help_strings.KeybindAction`),
+    /// each literal ending in one extra newline.
     pub fn docs(self) -> &'static str {
         match self {
             ActionKey::Ignore => {
@@ -1438,6 +1448,7 @@ Valid arguments are:
 
     Adjust the selection to the beginning or the end of the line
     respectively.
+
 "##
             }
             ActionKey::JumpToPrompt => {
@@ -1468,6 +1479,7 @@ Valid actions are:
 
     The default OS editor is determined by using `open` on macOS
     and `xdg-open` on Linux.
+
 "##
             }
             ActionKey::WriteScreenFile => {
@@ -1574,6 +1586,7 @@ Valid arguments:
     Creates a new split along the larger direction.
     For example, if the parent split is currently wider than it is tall,
     then a left-right split would be created, and vice versa.
+
 "##
             }
             ActionKey::GotoSplit => {
@@ -1940,6 +1953,7 @@ The value determines the crash location:
   - `render`
 
     Crash on the render thread for the focused surface.
+
 "##
             }
         }