Node's writeFileSync accepts an open file descriptor where it accepts a
path: it writes the data at the descriptor's current offset and leaves the
descriptor open, since closeSync owns its lifetime. The static fs surface
declared only the path forms, so `fs.writeFileSync(fd, text)` failed type
mapping — the compiler was missing a documented overload, not a
conversion.
Declare the (fd, data) and (fd, data, "utf8" | "utf-8") forms in the
node-fallback typings, route the call by the first argument's static f64
type to a new fs.writeFileSyncFd libfn, and implement it as
scr_fs_write_file_fd: a write loop over the whole string through the
descriptor-validating scr_fs_write_bytes, resuming short writes, with the
descriptor operation's errno shape on failure.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
