fix: pass process environ to Io backend, fixing AppDataDirUnavailable

runtime.init() was calling Threaded.init(gpa, .{}) without passing the
process environ. The default environ in InitOptions is .empty, so all
child processes spawned via std.process.run/spawn received zero
environment variables.

This caused 'mer dev', 'mer build', and 'mer update' to fail on Linux
with 'unable to resolve zig cache directory: AppDataDirUnavailable'
because the child zig process had no HOME variable.

Fix: accept environ as a parameter in runtime.init() and forward it to
the Io backend (Threaded or Evented). All callsites now pass
init.environ from the Init.Minimal entry point.

Fixes child process environment inheritance on all platforms.

Author: Revaycolizer

cli.zig

@@ -53,7 +53,7 @@ pub fn main(init: std.process.Init.Minimal) !void {
     const alloc = gpa.allocator();
 
     // Initialize std.Io runtime (Auto-selects Evented on Linux, Threaded elsewhere)
-    try runtime.init(alloc);
+    try runtime.init(alloc, init.environ);
     defer runtime.deinit();
 
     var arena_state: std.heap.ArenaAllocator = .init(std.heap.page_allocator);

examples/ui-showcase/src/main.zig

@@ -16,7 +16,7 @@ pub fn main(init: std.process.Init.Minimal) !void {
     const alloc = gpa.allocator();
 
     // Initialize std.Io runtime (Auto-selects Evented on Linux, Threaded elsewhere)
-    try runtime.init(alloc);
+    try runtime.init(alloc, init.environ);
     defer runtime.deinit();
 
     var arena_state: std.heap.ArenaAllocator = .init(std.heap.page_allocator);

src/main.zig

@@ -16,7 +16,7 @@ pub fn main(init: std.process.Init.Minimal) !void {
     const alloc = gpa.allocator();
 
     // Initialize std.Io runtime (Auto-selects Evented on Linux, Threaded elsewhere)
-    try runtime.init(alloc);
+    try runtime.init(alloc, init.environ);
     defer runtime.deinit();
 
     // 0.16: args come through init parameter.

src/runtime.zig

@@ -21,15 +21,15 @@ const use_evented = blk: {
 // Evented storage only exists when supported
 var evented: if (use_evented) std.Io.Evented else void = undefined;
 
-pub fn init(gpa: std.mem.Allocator) !void {
+pub fn init(gpa: std.mem.Allocator, environ: std.process.Environ) !void {
     if (use_evented) {
         // Linux: Use Evented (io_uring)
         evented = undefined;
-        try std.Io.Evented.init(&evented, gpa, .{});
+        try std.Io.Evented.init(&evented, gpa, .{ .environ = environ });
         io = evented.io();
     } else {
         // macOS/Other: Use Threaded (Evented has bugs or isn't available)
-        threaded = std.Io.Threaded.init(gpa, .{});
+        threaded = std.Io.Threaded.init(gpa, .{ .environ = environ });
         io = threaded.io();
     }
 }

src/runtime_threaded.zig

@@ -6,13 +6,13 @@ const std = @import("std");
 /// This lets us flip between Threaded (today) and Evented (tomorrow for io_uring)
 /// without touching call sites.
 ///
-/// Initialize once in main(): runtime.init(gpa);
+/// Initialize once in main(): runtime.init(gpa, init.environ);
 /// Clean up on exit: defer runtime.deinit();
 pub var threaded: std.Io.Threaded = undefined;
 pub var io: std.Io = undefined;
 
-pub fn init(gpa: std.mem.Allocator) void {
-    threaded = std.Io.Threaded.init(gpa, .{});
+pub fn init(gpa: std.mem.Allocator, environ: std.process.Environ) void {
+    threaded = std.Io.Threaded.init(gpa, .{ .environ = environ });
     io = threaded.io();
 }
 

tools/codegen.zig

@@ -10,7 +10,7 @@ pub fn main() !void {
     const alloc = gpa.allocator();
 
     // Initialize std.Io runtime (Auto-selects Evented on Linux, Threaded elsewhere)
-    try runtime.init(alloc);
+    try runtime.init(alloc, .empty);
     defer runtime.deinit();
 
     // Each entry stores the full relative path from the project root.