fix(runtime): gate io_uring/Evented behind stdlib bug flag for 0.16.0 CI

Zig 0.16.0's release stdlib has a known bug in std/Io/Uring.zig where
Dir.OpenError and Dir.RealPathFileError are missing
error.ReadOnlyFileSystem, but the Uring vtable forwards it via
`else => |e| return e`. Calling `Evented.init().io()` forces analysis of
every Uring vtable function and triggers compile errors on stock 0.16.0
in CI.

Add `stdlib_evented_works = false` const guard. The whole fallback
architecture is preserved — flipping it back to true re-enables the
Evented (io_uring) path with no other code changes once the toolchain
pin moves past 0.16.0. Docs updated to reflect current state.

Co-Authored-By: Rach Pradhan <pradhan.rach4@gmail.com>

Author: devin-ai-integration[bot]

README.md

@@ -369,16 +369,25 @@ The `worker/worker.js` shim handles the fetch event and passes requests to the W
 
 ### Runtime selection (io_uring)
 
-On Linux, merjs tries to use **io_uring** (`std.Io.Evented`) for max throughput.
-If io_uring isn't available — old kernel, restricted seccomp, sandboxed runtime —
-the framework **automatically falls back** to a thread-pool backend so the same
-binary boots everywhere. You'll see one of these log lines at startup:
+`src/runtime.zig` exposes a single `runtime.io` instance and a `Backend` enum
+(`evented` / `threaded`). On startup, you'll see:
 
 ```
-info(runtime): io backend: Evented (io_uring)
 info(runtime): io backend: Threaded (blocking syscalls)
 ```
 
+The framework is wired to opportunistically pick **Evented** (Linux io_uring)
+when the toolchain supports it, and gracefully **fall back to Threaded** if
+io_uring init fails (old kernel, restricted seccomp, sandboxed container, …) so
+the same binary boots on every host.
+
+While merjs is pinned to Zig **0.16.0**, the io_uring path is gated off — the
+release tarball's `std/Io/Uring.zig` has a known compile-time bug (missing
+`error.ReadOnlyFileSystem` in `Dir.OpenError`/`Dir.RealPathFileError`) that
+prevents Evented from being analyzed at all. Flip
+`stdlib_evented_works = true` in `src/runtime.zig` once the toolchain pin
+moves past 0.16.0; no other code changes needed.
+
 ---
 
 ## How It Works

src/runtime.zig

@@ -23,7 +23,22 @@ pub var io: std.Io = undefined;
 var active: Backend = .threaded;
 
 // Evented is only available on platforms where the stdlib provides it.
+//
+// NOTE: Zig 0.16.0's release stdlib ships a known bug in `std/Io/Uring.zig`
+// (the Linux/io_uring Evented backend) — `Dir.OpenError` and
+// `Dir.RealPathFileError` don't include `error.ReadOnlyFileSystem`, but the
+// Uring vtable's `else => |e| return e` branch tries to forward it. Merely
+// calling `Evented.init().io()` forces analysis of every Uring vtable
+// function and triggers compile errors. The bug is fixed in zig master but
+// not in the 0.16.0 release tarball that CI uses.
+//
+// Until the toolchain pin moves past 0.16.0, force Threaded everywhere so the
+// binary actually builds on stock 0.16.0. The whole fallback architecture
+// below is preserved — flipping `stdlib_evented_works` to `true` re-enables
+// the Evented (io_uring) path with no other code changes.
+const stdlib_evented_works = false;
 const evented_supported = blk: {
+    if (!stdlib_evented_works) break :blk false;
     if (!@hasDecl(std.Io, "Evented")) break :blk false;
     if (std.Io.Evented == void) break :blk false;
     // Only attempt Evented on Linux (io_uring). Other Evented backends are