fix: replace hardcoded locale.h constants with build-system TranslateC (#11920)

Replace hardcoded locale.h constants and extern function declarations
with build-system TranslateC, following the same pattern as pty.c.

This fixes LC_ALL being hardcoded to 6 (the musl/glibc implementation
value), which is implementation-defined and differs on Windows MSVC
(where LC_ALL is 0), causing `setlocale()` to crash with an invalid
parameter error.

## Changes

- Added `src/os/locale.c` — includes `locale.h` for TranslateC
- Added TranslateC step in `src/build/SharedDeps.zig` (same pattern as
pty.c)
- Replaced hardcoded constants and extern declarations in
`src/os/locale.zig` with `@import("locale-c")`

## AI disclosure

Claude Code was used to assist with debugging and identifying this
issue.

src/build/SharedDeps.zig

@@ -135,6 +135,24 @@ pub fn add(
     // Every exe needs the terminal options
     self.config.terminalOptions().add(b, step.root_module);
 
+    // C imports for locale constants and functions
+    {
+        const c = b.addTranslateC(.{
+            .root_source_file = b.path("src/os/locale.c"),
+            .target = target,
+            .optimize = optimize,
+        });
+        if (target.result.os.tag.isDarwin()) {
+            const libc = try std.zig.LibCInstallation.findNative(.{
+                .allocator = b.allocator,
+                .target = &target.result,
+                .verbose = false,
+            });
+            c.addSystemIncludePath(.{ .cwd_relative = libc.sys_include_dir.? });
+        }
+        step.root_module.addImport("locale-c", c.createModule());
+    }
+
     // C imports needed to manage/create PTYs
     switch (target.result.os.tag) {
         .freebsd,

src/os/locale.c

@@ -0,0 +1 @@
+#include <locale.h>

src/os/locale.zig

@@ -210,9 +210,10 @@ fn preferredLanguageFromCocoa(
     return slice[0 .. slice.len - 1 :0];
 }
 
-const LC_ALL: c_int = 6; // from locale.h
-const LC_ALL_MASK: c_int = 0x7fffffff; // from locale.h
-const locale_t = ?*anyopaque;
-extern "c" fn setlocale(category: c_int, locale: ?[*]const u8) ?[*:0]u8;
-extern "c" fn newlocale(category: c_int, locale: ?[*]const u8, base: locale_t) locale_t;
-extern "c" fn freelocale(v: locale_t) void;
+const c = @import("locale-c");
+const LC_ALL: c_int = c.LC_ALL;
+const LC_ALL_MASK: c_int = c.LC_ALL_MASK;
+const locale_t = c.locale_t;
+const setlocale = c.setlocale;
+const newlocale = c.newlocale;
+const freelocale = c.freelocale;