fs: /bin alias for the system volume

ADR 0006 deferred root-path unification because it "needs VFS
bind/alias machinery".  Implement the /bin half: paths whose first
component is `bin` are rewritten to the first mounted volume whose
root has a \bin directory, so `ls /bin/cli`, `cat /bin/cli/x.efi` and
a binary's own fs_resolve("/bin/cli/x.efi") work without knowing the
volume UID.

The alias is transparent — it creates no node, so `ls /` keeps its old
listing and /bin simply does not exist when no system volume is
mounted — and allocates nothing for non-matching paths.  exec::resolve
keeps its per-file volume scan (stricter for multiple volumes).

Update ADR 0006 to record the decision and what remains deferred
(\etc and friends at the root).

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