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).

1 files changed, 11 insertions, 1 deletions

diff --git a/docs/adr/0006-executable-binaries-and-bin-tree.md b/docs/adr/0006-executable-binaries-and-bin-tree.md
index 17109c4..f3de95f 100644
--- a/docs/adr/0006-executable-binaries-and-bin-tree.md
+++ b/docs/adr/0006-executable-binaries-and-bin-tree.md
@@ -48,6 +48,14 @@ and so a launcher can enumerate launchable apps by listing a directory.
 - **Resolution rule.**  For name N: run `\bin\<kind>\N` from the first
   mounted volume that has it; else the built-in N; else "unknown
   command".  No volume is privileged; no boot-volume identity exists.
+- **`/bin` is a VFS path alias.**  User paths whose first component is
+  `bin` are rewritten (in `fs::resolve`) to the first mounted volume
+  whose root has `\bin` — 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 (`ls /` does
+  not list `bin`), and without a system volume mounted `/bin` does not
+  exist.  It covers `/bin` only — the rest of root-path unification
+  stays deferred, but the bind/alias machinery it needed now exists.
 - **Kind = launch surface.**  `/bin/cli` holds programs that run inside a
   terminal session; `/bin/gui` holds programs launchable with no terminal
   (they own their windows).  A future launcher enumerates `/bin/gui`.
@@ -91,7 +99,9 @@ and so a launcher can enumerate launchable apps by listing a directory.
   through FAT write-back persists across reboots, and the image doubles
   as the distribution artifact.
 - **Boot-volume identity / root-path unification** (`\etc` visible at
-  `/etc`) — deferred; needs VFS bind/alias machinery.
+  `/etc`) — deferred; needed VFS bind/alias machinery.  The `/bin` half
+  landed as a path alias (see the resolution-rule section); `\etc`,
+  `\bzImage` etc. at the root stay deferred.
 - **A third "TUI" binary kind for rsdosedit** — rejected; `edit` is
   classified CLI (it lives in a terminal session) and migrates only after
   the CLI ABI grows raw-mode surface access.  Until then it stays a