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-terminal/src/page.rs

@@ -977,6 +977,21 @@ impl Page {
     pub fn lookup_hyperlink(&self, index: usize) -> Option<u16> {
         self.hyperlink_map.get(&index).copied()
     }
+    /// Bytes of string storage currently in use (Zig:
+    /// `string_alloc.usedBytes`); used by the inspector.
+    pub fn string_used_bytes(&self) -> usize {
+        let used: usize = self
+            .string_alloc
+            .used
+            .iter()
+            .map(|w| w.count_ones() as usize)
+            .sum();
+        used * self.string_alloc.chunk
+    }
+    /// Total bytes of string storage (Zig: `string_alloc.capacityBytes`).
+    pub fn string_capacity_bytes(&self) -> usize {
+        self.string_alloc.chunks * self.string_alloc.chunk
+    }
     /// Number of hyperlink cells.
     pub fn hyperlink_count(&self) -> usize {
         self.hyperlink_map.len()