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/mod.rs

@@ -498,10 +498,8 @@ pub const BACKWARDS_COMPATIBLE_KEYS: &[(&str, TriggerKey)] = {
 /// The codepoint we use for comparisons. Case folding can result
 /// in more codepoints so we need to use a 3 element array.
 ///
-/// DEVIATION: Zig uses uucode's full case folding table. Here we use
-/// `char::to_lowercase` (single-char results only) plus a few simple-folding
-/// special cases; multi-codepoint foldings return the codepoint as-is exactly
-/// like Zig does.
+/// Uses the full Unicode case folding table (uucode's `case_folding_full`,
+/// generated by `ghostty-unicode`).
 fn folded_codepoint(cp: u32) -> [u32; 3] {
     // ASCII fast path
     if let Some(c) = char::from_u32(cp)
@@ -511,24 +509,7 @@ fn folded_codepoint(cp: u32) -> [u32; 3] {
     }
 
     // Unicode slow path. Case folding can result in more codepoints.
-    // If more codepoints are produced then we return the codepoint
-    // as-is which isn't correct but until we have a failing test
-    // then I don't want to handle this.
-    let Some(c) = char::from_u32(cp) else {
-        return [cp, 0, 0];
-    };
-    // Simple-fold differences from lowercase.
-    match c {
-        '\u{00B5}' => return [0x03BC, 0, 0],
-        '\u{017F}' => return ['s' as u32, 0, 0],
-        '\u{03C2}' => return [0x03C3, 0, 0],
-        _ => {}
-    }
-    let mut it = c.to_lowercase();
-    match (it.next(), it.next()) {
-        (Some(l), None) => [l as u32, 0, 0],
-        _ => [cp, 0, 0],
-    }
+    ghostty_unicode::case_folding::fold_full(cp)
 }
 
 impl Trigger {