fix-deviant(termio): resolve bare program names on PATH on Windows

Intentional deviation from Ghostty 1.3.1 (DEVIANT_FIXES.md row 39).
Zig's Command.startWindows (Command.zig:261,347-358) passes the command
path as CreateProcessW's lpApplicationName, which is taken literally:
a partial name is completed with the current directory only, never the
search path. A bare `cmd.exe` (e.g. `command = direct:cmd.exe` or
`-e cmd.exe`) therefore fails to start in Ghostty on Windows.

A bare program name (no drive or separator) is now looked up on PATH,
appending .exe when there is no extension as CreateProcess does;
anything else or a failed lookup is passed through unchanged. Verified
under Wine by the exec_windows ConPTY tests.

Test Plan:
- cargo test -p ghostty-termio resolve_application_searches_path_for_bare_names
- just test-windows-wine

docs/rewrite/DEVIANT_FIXES.md

@@ -64,6 +64,7 @@ when this file was written.
 | 36 | Font (wasm/browser Canvas): atlas pixel format | `font/face/web_canvas.zig` (atlas write branch on `.rgba`) | The branch tests a stale `.rgba` format against an enum that contains `.bgra`, so BGRA atlases are never converted. | The BGRA atlas conversion is implemented. No claim of equivalence with a real browser (not testable here). | d89691b | `ghostty-font` web_canvas tests (compile/Node level) |
 | 37 | Termio: subprocess cleanup | `termio/Exec.zig:1188-1224` (`getpgid`) | Loops forever while `getpgid(child) == our pgid`, i.e. also when the child died before `setsid` and is an unreaped zombie (its pgid stays ours), hanging the surface close. | Still waits without a cap (no 5 s limit) while the child is alive, but stops (no SIGHUP, nothing to kill) once `waitid(WNOWAIT)` shows the child exited. | uncommitted (P3) | `getpgid_gives_up_on_exited_child_without_setsid`, `getpgid_waits_for_late_setsid` (`ghostty-termio/src/exec.rs`) |
 | 38 | Config (Windows): absolute theme paths | `config/Config.zig:9788-9799` (`Theme.parseCLI`) | Any value containing `,`, `=` or `:` is parsed as a `light:..,dark:..` pair, and a Windows absolute path (`C:\\themes\\x`, `\\\\?\\C:\\...`) always contains a colon, so `theme = <absolute path>` is rejected on Windows (found by running the config tests under Wine). | On Windows a value that starts like an absolute Windows path (drive letter + `:` + separator, or `\\\\`) is a single theme; the pair syntax is unchanged (quote backslashes). Other platforms are unchanged. | uncommitted (agent WINE) | `windows_absolute_path_detection`, `theme_windows_absolute_path_and_pair_with_paths` (`ghostty-config/src/types_b.rs`), `theme_loading` under Wine |
+| 39 | Termio (Windows): bare program names | `Command.zig:261,347-358` (`startWindows`, `CreateProcessW(application_w, ...)`) with `termio/Exec.zig:1004` (`.path = self.args[0]`) | `lpApplicationName` is never searched on `PATH` (a partial name only completes against the current directory), so a `direct:` command / `-e` argv[0] such as `cmd.exe` fails with `FileNotFound`; only the default shell works (it is wrapped as `%WINDIR%\\System32\\cmd.exe /C ...`). | A name without drive/separator is looked up on `PATH` (`.exe` implied) before `CreateProcessW`; failures keep the original name (same error as Zig). | uncommitted (agent WINE) | `resolve_application_searches_path_for_bare_names`, `bare_program_name_starts_via_path_search` (`ghostty-termio/src/command_windows.rs`), `echo_output_reaches_terminal_and_exit_is_reported` (`tests/exec_windows.rs`, Wine) |
 
 ## Upstream quirks deliberately kept (not fixed)