lib: fmt fprint family over io.handle; remove the fdsink workaround (#5)

Graduates the fmt fprint family (fprint/fprintf/fprintln/fprintfln + internal putbytes/writeone/format*) from io.stream to io.handle, so a file (fd) prints directly through io.write's file-arm (commit-1). Removes the fdsink placeholder -- the fake-stream-vtable-over-os.write shim that stood in for the missing handle. The 8 stdio wrappers route over os.STD{OUT,ERR}_FILENO (new i32 filenos in lib/os; os is the import floor, so it can't hold an io.file-typed handle like Hare's os::stdout_file -- consumers cast i32 to io.file). Migrates the fd-shim sentinel tests 777/780/781 to fprint-over-handle as their headers designed, cstage-only per the pre-existing #209 (fmt is wwstage-uncompilable). Regenerates the 6 os-embedding combined.ww.

selfhost/cmd/w6a/main.combined.ww

@@ -189,6 +189,17 @@ export fn exit(code: i32) void = {
 export def PATH_MAX: i32 = 4096;
 let pathbuf: [4096]u8;
 
+// ref/hare/sys/+linux/types.ha:886-888. ww folds `sys` into `os`, so the
+// std fd NUMBERS live here (the sys role). Typed i32, NOT io.file as in
+// Hare's os::stdout_file (ref/hare/os/+linux/stdfd.ha:28): Hare's `os`
+// imports `io`, but ww's `os` is the import floor and must never import
+// io (lib/CLAUDE.md) — so the io.file/io.handle binding can't live here.
+// Consumers (lib/fmt's stdio wrappers) cast i32→io.file at the use site,
+// where the handle layer is already in scope.
+export def STDIN_FILENO:  i32 = 0;
+export def STDOUT_FILENO: i32 = 1;
+export def STDERR_FILENO: i32 = 2;
+
 fn kpath(p: str) *u8 = {
 	if (p.len + 1 >= PATH_MAX) { return nil: *u8; };	// ENAMETOOLONG
 	let i: i32 = 0;