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/tests.rs

@@ -1507,6 +1507,20 @@ fn trigger_hash_folds_case() {
     assert!(!uni('a').equal(&uni('A')));
 }
 
+#[test]
+fn trigger_full_unicode_case_folding() {
+    // Simple folds beyond to_lowercase: micro sign, long s, final sigma.
+    assert!(uni('\u{00B5}').folded_equal(&uni('\u{03BC}')));
+    assert!(uni('\u{017F}').folded_equal(&uni('s')));
+    assert!(uni('\u{03C2}').folded_equal(&uni('\u{03C3}')));
+    // Multi-codepoint full folds: capital sharp s folds to "ss", as does
+    // U+00DF itself.
+    assert!(uni('\u{1E9E}').folded_equal(&uni('\u{00DF}')));
+    assert_eq!(uni('\u{1E9E}').hash(), uni('\u{00DF}').hash());
+    // Non-equivalent codepoints stay distinct.
+    assert!(!uni('\u{00DF}').folded_equal(&uni('s')));
+}
+
 #[test]
 fn trigger_display_roundtrip() {
     for s in ["super+ctrl+alt+shift+a", "catch_all", "ctrl+arrow_up", "+"] {