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

docs/rewrite/ARCHITECTURE.md

@@ -21,6 +21,7 @@ WASM, Windows and the embedded C API (`libghostty`) are out of scope.
 | `ghostty-renderer` | renderer/ generic, OpenGL (+ opengl/*), cell, cursor, image, link, size, State, Thread, Overlay, shadertoy, row, shaders/glsl | config, font, terminal |
 | `ghostty-termio` | termio/*, pty.zig, Command.zig, shell-integration resources | config, terminal, os |
 | `ghostty-cli` | cli/* actions (+list-themes, +show-config, +list-keybinds, ...) | config, input, os, font |
+| `ghostty-inspector` | inspector/* (Dear ImGui terminal inspector: widgets, key + VT event recording) | input, renderer, terminal, vt, font, `dear-imgui-rs` |
 | `ghostty-core` | App.zig, Surface.zig, surface_mouse.zig, apprt/action.zig, apprt/structs.zig, apprt/surface.zig, global.zig | everything below |
 | `ghostty-apprt` | apprt/action.zig, structs.zig, surface.zig, ipc.zig (core↔runtime contract types) | vt, input, config, datastruct |
 | `ghostty-apprt-gtk` | apprt/gtk/* | core, config, input |
@@ -75,7 +76,7 @@ WASM, Windows and the embedded C API (`libghostty`) are out of scope.
 ## Out of scope / deferred
 
 - macOS (Swift app, Metal, CoreText), Windows, WASM, libghostty C API.
-- The ImGui inspector is deferred until everything else works (Phase 7).
+- (The ImGui inspector was deferred and is now ported: `docs/rewrite/notes/ghostty-inspector.md`.)
 
 ## Intentional omissions (decided by the orchestrator)