fs: use the tracing channels directly

The fs modules now hold the TracingChannel instances themselves, and
each publish site builds its own context literal, so the shape of the
context is fixed per site instead of computed from a field table on
every publish. The check at the start of each function is
`hasSubscribers` on the held channel.

The trace helpers call the traced function again with a private `this`,
so that the second entry skips the check. This replaces the re-entry
flag and the wrapper that consumed it. No traced function uses `this`
for anything else.

Refs: #66575 (review)
Assisted-by: claude:fable-5.1
Signed-off-by: Tim Fish <tim@timfish.uk>

Author: timfish