fix: systematic byte fidelity for config strings and RV7 gaps

Resolves the 9 gap groups in docs/rewrite/notes/review-rv7-codex.md and
audits every string-typed config field against its consumers
(field -> consumers -> decode status table in final-Z9.md):
- Windows EnvMap keys use a port of Zig's nls.zig uppercase table
  (upcaseW); Windows locale init follows Zig's chain (setlocale +
  _putenv_s); malformed WTF-8 now fails process launch conversions like
  Zig instead of substituting U+FFFD; kitty file paths convert WTF-8.
- bash --rcfile/--init-file, configured TERM and inherited GTK LANG
  keep their original bytes.
- C API clipboard completion and ghostty_surface_binding_action take
  bytes; complete_clipboard_request is byte-typed through the core.
- text:/csi:/esc: bindings, enquiry-response, input = path:, embedded
  initial_input, search needles, titles, link regexes, clipboard
  codepoint map, command palette text and GTK font/namespace values
  decode the escape encoding before use.

Remaining by design: D-Bus a{ss} values and +new-window arguments are
UTF-8 by D-Bus type. C API key/preedit text follows separately.

Test Plan:
- cargo test --workspace
- just check-macos (both), just check-windows, just check-wasm

crates/ghostty-termio/tests/exec_integration.rs

@@ -103,7 +103,10 @@ fn start(command: ConfigCommand, tweak: impl FnOnce(&mut Config)) -> Fixture {
     let mut config = base_config();
     tweak(&mut config);
 
-    let mut ecfg = exec::Config::new(ghostty_os::env::get_env_map(), "xterm-256color");
+    let mut ecfg = exec::Config::new(
+        ghostty_os::env::get_env_map(),
+        ghostty_os::osbytes::EncodedOs::from_plain_text("xterm-256color"),
+    );
     ecfg.command = Some(command);
     ecfg.shell_integration = ShellIntegration::None;
     // The default config has `linux-cgroup = single-instance` +
@@ -360,9 +363,9 @@ fn initial_input_is_written_after_start() {
 #[test]
 fn missing_input_file_shows_error_and_thread_keeps_draining() {
     let mut f = start(sh("true"), |c| {
-        c.input
-            .list
-            .push(ReadableIO::Path("/nonexistent/ghostty-input".into()));
+        c.input.list.push(ReadableIO::Path(
+            ghostty_os::osbytes::EncodedOs::from_plain_text("/nonexistent/ghostty-input"),
+        ));
     });
     f.wait_text("A configured `input` path was not found");
     assert!(
@@ -575,7 +578,10 @@ fn change_config_message_updates_palette_and_colors() {
 fn process_output_manual_api() {
     // Termio can also be fed manually (no child involved).
     let config = base_config();
-    let mut ecfg = exec::Config::new(ghostty_os::EnvMap::new(), "xterm-256color");
+    let mut ecfg = exec::Config::new(
+        ghostty_os::EnvMap::new(),
+        ghostty_os::osbytes::EncodedOs::from_plain_text("xterm-256color"),
+    );
     ecfg.command = Some(sh("true"));
     let (tx, _rx) = unbounded();
     let termio = Termio::init(Options {

crates/ghostty-termio/tests/flatpak_exec.rs

@@ -108,7 +108,10 @@ fn start(command: ConfigCommand, tweak: impl FnOnce(&mut Config)) -> Fixture {
     let mut config = base_config();
     tweak(&mut config);
 
-    let mut ecfg = exec::Config::new(ghostty_os::env::get_env_map(), "xterm-256color");
+    let mut ecfg = exec::Config::new(
+        ghostty_os::env::get_env_map(),
+        ghostty_os::osbytes::EncodedOs::from_plain_text("xterm-256color"),
+    );
     ecfg.command = Some(command);
     ecfg.shell_integration = ShellIntegration::None;
     // The default config has `linux-cgroup = single-instance` +