lib: port errors.errno + os errno/strerror sys-layer (#6)

Hare-faithful port of errors::errno (ref/hare/errors/{rt,common,opaque}.ha): the 13 named common error conditions, opaque_data/opaque_ (the type-erased tail whose strerror fn-ptr defers to os.strerror), and errno(os.errno) error mapping the ~12 mapped errnos to named conditions and wrapping the unmapped tail in opaque_. The raw errno type (!i32, kernel-int width, distinct from oserror's !i64 negative raw return), the E* constants, and the strerror message table live in lib/os: ww folds Hare's sys role into os, so os is the import floor that lib/io and lib/errors build on -- documented in lib/CLAUDE.md (os never imports io or errors). errors.error is explicitly enumerated, matching Hare; the ...errors::error spread is only io.error's (blocked by #199b). Prereq for post-eFinal #5's faithful io error mapping; retires the nomem-collapse interim. Adds errnotest (mapping / opaque-tail / strerror) + test/wcc/902_errno_run. Landing required two wwstage cgen fixes (#9 struct-variant-large-union return, #11 deref-store alias narrow). Divergences cited at-site: bare-type-name return -> let+return; switch fall-through vs Hare's exhaustiveness-only default; opaque_ const dropped.

selfhost/cmd/w6a/main.combined.ww

@@ -265,6 +265,57 @@ export fn lseek(fd: i32, off: i64, w: whence) i64 = {
 // errors::errno carried inside io::error.
 export type oserror = !i64;
 
+// errno — the raw Linux errno as a positive code (ref/hare/sys/+linux/
+// errno.ha:5, `errno = !int`). ww folds Hare's `sys` role into os
+// (lib/CLAUDE.md), so the sys::errno machinery lands here. Spelled i32
+// rather than int: Linux errnos are kernel ints (32-bit), keeping os's
+// kernel-facing surface uniformly i32. Distinct from [[oserror]] (!i64,
+// the syscall's *negative* raw return) — the two model different
+// things, so they are not unified; the negative→positive normalization
+// lives at the oserror→errors.error boundary in those callers.
+export type errno = !i32;
+
+// Mapped errno values, ref/hare/sys/+linux/errno.ha:559-682. Positive,
+// matching Hare's defs (the kernel returns -N; the wrap-to-positive is
+// the caller's concern). Subset: exactly the errnos [[errors.errno]]
+// maps to a named condition; grow as callers surface more.
+export def ENOENT:       errno = 2;
+export def EINTR:        errno = 4;
+export def EAGAIN:       errno = 11;
+export def EACCES:       errno = 13;
+export def EBUSY:        errno = 16;
+export def EEXIST:       errno = 17;
+export def EINVAL:       errno = 22;
+export def EOVERFLOW:    errno = 75;
+export def ENETUNREACH:  errno = 101;
+export def ETIMEDOUT:    errno = 110;
+export def ECONNREFUSED: errno = 111;
+export def ECANCELED:    errno = 125;
+
+// strerror — human-readable text for an [[errno]] (Hare's
+// sys::strerror, ref/hare/sys/+linux/errno.ha:18). FAITHFUL MINIMAL
+// SUBSET: the mapped errnos above plus a generic fallback; grow the
+// switch as callers surface more (lib/CLAUDE.md documented-subset, not
+// a workaround). Messages verbatim from the reference. Hare's
+// unknown_errno formats the numeric value; that is deferred.
+export fn strerror(err: errno) str = {
+	switch (err) {
+	case ENOENT:       return "No such file or directory";
+	case EINTR:        return "Interrupted system call";
+	case EAGAIN:       return "Resource temporarily unavailable";
+	case EACCES:       return "Permission denied";
+	case EBUSY:        return "Device or resource busy";
+	case EEXIST:       return "File exists";
+	case EINVAL:       return "Invalid argument";
+	case EOVERFLOW:    return "Value too large for defined data type";
+	case ENETUNREACH:  return "Network is unreachable";
+	case ETIMEDOUT:    return "Connection timed out";
+	case ECONNREFUSED: return "Connection refused";
+	case ECANCELED:    return "Operation canceled";
+	};
+	return "Unknown error";
+};
+
 // filesize — byte length of an open fd via lseek-to-end-and-back.
 export fn filesize(fd: i32) (i64 | oserror) = {
 	let end: i64 = lseek(fd, 0i64, whence.END);