fix(windows): prefer pwsh for the default pane shell (#4297)

* fix(windows): prefer pwsh for the default pane shell

Windows panes launched Windows PowerShell 5.1 (powershell.exe) whenever [terminal] default_shell was unset, ignoring PowerShell 7 even when pwsh.exe resolved on PATH. Resolve the unset default against PATH and prefer pwsh.exe, falling back to the inbox powershell.exe. An explicit default_shell still wins.

* fix(windows): validate pwsh before using it as the default shell

portable-pty resolves the configured shell with Path::exists and passes that path to CreateProcessW, which does not fall through to later PATH entries. Selecting an invalid pwsh.exe would therefore break new panes instead of falling back. Only prefer a pwsh.exe that starts with the PE MZ magic, and return its path so the validated binary is the one launched.

* fix(windows): validate the full PE header before preferring pwsh

Checking only the DOS MZ signature still accepted truncated images, DLLs, and foreign-architecture binaries. Because portable-pty passes the resolved path straight to CreateProcessW without trying later PATH entries, validate the PE signature, the COFF header (machine, executable-image bit, not a DLL, section count), and the optional header before preferring pwsh.exe.

* fix(windows): accept native ARM64 pwsh and validate the section table

Herdr ships an x64 Windows build that also runs on Windows ARM64 under x64 emulation, so cfg!(target_arch) cannot tell whether a native ARM64 pwsh.exe is launchable. Accept every executable machine type Windows supports instead of rejecting ARM64 on the x86_64 build. Also bound-check the section table (40 bytes per declared section) after the optional header so a truncated table falls back instead of being selected.

* fix(windows): detect the native host machine for pwsh compatibility

Machine compatibility was derived from cfg!(target_arch), which reports the emulated x64 process on Windows ARM64, and then briefly accepted ARM64 on every host. Read the native machine with IsWow64Process2 in the platform layer instead: ARM64 Windows accepts ARM64/x64/x86 images, x64 accepts x64/x86, and x86 accepts only x86.

1 files changed, 28 insertions, 5 deletions

diff --git a/src/platform/windows.rs b/src/platform/windows.rs
index 9e39a715..cdf4acfc 100644
--- a/src/platform/windows.rs
+++ b/src/platform/windows.rs
@@ -350,11 +350,11 @@ use windows_sys::{
             },
             Ole::{CF_DIB, CF_DIBV5, CF_UNICODETEXT},
             Threading::{
-                GetCurrentProcess, GetExitCodeProcess, GetProcessTimes, OpenProcess, OpenThread,
-                QueryFullProcessImageNameW, ResumeThread, TerminateProcess, CREATE_NO_WINDOW,
-                CREATE_SUSPENDED, DETACHED_PROCESS, PROCESS_BASIC_INFORMATION,
-                PROCESS_QUERY_INFORMATION, PROCESS_QUERY_LIMITED_INFORMATION, PROCESS_VM_READ,
-                THREAD_SUSPEND_RESUME,
+                GetCurrentProcess, GetExitCodeProcess, GetProcessTimes, IsWow64Process2,
+                OpenProcess, OpenThread, QueryFullProcessImageNameW, ResumeThread,
+                TerminateProcess, CREATE_NO_WINDOW, CREATE_SUSPENDED, DETACHED_PROCESS,
+                PROCESS_BASIC_INFORMATION, PROCESS_QUERY_INFORMATION,
+                PROCESS_QUERY_LIMITED_INFORMATION, PROCESS_VM_READ, THREAD_SUSPEND_RESUME,
             },
         },
         UI::{
@@ -386,6 +386,29 @@ const FOREGROUND_SELECTION_CACHE_CAPACITY: usize = 1_024;
 const FOREGROUND_SELECTION_CACHE_RETENTION: Duration = Duration::from_secs(60);
 const PANE_RUNTIME_MARKER_ENV_VAR: &str = "HERDR_PANE_RUNTIME_ID";
 
+/// Native processor architecture of the Windows host as an
+/// `IMAGE_FILE_MACHINE_*` value. `IsWow64Process2` reports the native machine
+/// even when an x64 Herdr runs under emulation on Windows ARM64, which the
+/// build target alone cannot reveal.
+pub(crate) fn native_machine_type() -> u16 {
+    let mut process_machine = 0u16;
+    let mut native_machine = 0u16;
+    // SAFETY: `GetCurrentProcess` returns a pseudo-handle and both out-pointers
+    // are valid for the duration of the call.
+    let ok = unsafe {
+        IsWow64Process2(
+            GetCurrentProcess(),
+            &mut process_machine,
+            &mut native_machine,
+        )
+    };
+    if ok != 0 && native_machine != 0 {
+        native_machine
+    } else {
+        process_machine
+    }
+}
+
 pub(crate) fn terminal_title_for_presentation(title: &str) -> &str {
     title.strip_prefix("Administrator: ").unwrap_or(title)
 }