unix: a bound socket is a file

Gate 1, item 3, and the last of foundations.md.

bind() creates a T_SOCK node at the path; connect() resolves the path and looks
for the socket bound to **that inode**. Matching on the inode rather than on
the path *string* is the part that matters: the name used to live only in the
socket table, so two spellings of one path missed each other, a relative path
never matched an absolute one, and there was nothing to stat() or unlink().
An inode is the thing a path resolves *to*, so connect() can walk the path
however it likes and arrive at the same socket.

    # ls -l /tmp
    srwxr-xr-x 1 root root 0 2026-09-22 08:04 socktest

The node is a marker, not the binding. It holds no data and the live
association stays in the socket table, so a socket file left by a crash is
what Linux leaves behind -- a dead file that connect() refuses until something
binds again -- and unlink()ing a live socket's path does not disturb anyone
already connected.

EADDRINUSE comes from the *directory* rather than from a scan of the table,
deliberately: a stale socket file has to refuse a rebind for the same reason a
running server's does, or two servers both believe they own it. The creation
lives in sys_bind() and not in unix.c, because that is the layer with both --
kernel/net/unix.c knows about sockets and has no business calling create().

T_SOCK is a compat addition to SchFS by the rule in schfs.h: a driver that does
not know SCHFS_T_SOCK maps it to a zero-length regular file, so nothing goes
stale and nothing becomes unreadable.

cmd/socktest section 15 asserts the plan's own wording -- bind, stat, see an
`s`, unlink -- plus the two a name table could never fail: a second bind to a
live path is refused, and connecting to a path that is a *regular file* is
refused rather than matched by name.

**And the semantics being real broke socktest on its second run**, correctly:
the socket files persisted and every bind() hit EADDRINUSE. Every real server
unlinks before binding; socktest now does too, which is the change the new
behaviour demands rather than a workaround for it.

Chasing that turned up something worse. `socktest` reported rc=1 with every
check passing, and so did every other first-party program:

  - **lib/start.S called main and threw %eax away.** sys_exit() reads the
    status from %rdi and nothing put it there, so every program exited with
    whatever that register happened to hold. Every test suite ends in
    `return failures ? 1 : 0` and none of it reached the shell.
  - **exit(void) passed no status either**, for the same reason: the
    declaration takes no argument, so %rdi was the caller's litter.

`if socktest; then` works now, and a failing command reports 1 -- checked both
ways, because a fix that makes everything return 0 would look identical from
the success side.

Also factored: unlink_path() out of sys_unlink, so a failed bind can take its
own node back out without going through the syscall entry, and create_sock()
as a named door rather than exporting create() with an arbitrary type.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/start.S

@@ -39,6 +39,18 @@ _start:
     andq    $-16, %rsp
 
     callq   main                /* main(%rdi = argc, %rsi = argv) */
-    callq   exit                /* exit() is noreturn, but be certain */
+
+    /* main's return value is the exit status, and it used to be thrown away.
+     *
+     * sys_exit() reads the status from %rdi, and nothing put it there -- so
+     * every first-party program exited with whatever happened to be in that
+     * register, and `if socktest; then` could not work for any of them. The
+     * test suites all end in `return failures ? 1 : 0` and none of it reached
+     * the shell.
+     *
+     * %eax, not %rax: the status is the low eight bits, and a `return 0` from
+     * a C function leaves only %eax defined. */
+    movl    %eax, %edi
+    callq   exit_group          /* noreturn, but be certain */
 1:  hlt
     jmp     1b