Plan 9 idiom for the control filesystem, and the regressions a624a56 left

The 9P tree stops being a command language wearing a filesystem. /new
created a pane as a side effect of a *read*; it is now Tcreate in /pane,
with Tremove to close, which cloud9's engine has always supported and the
editor never declared: tree.zig now says
`features = .{ .create = true, .remove = true }`. Eleven pane ctl verbs
become files that can be read as well as written -- dot, limit, dirty,
mark, scroll, look, exec -- leaving ctl with `get`, the one verb no file
would say better. Root /ctl splits into a read-only /status and the
/look and /exec files whose write IS the click. stat carries real sizes
where it used to answer 0, and qid versions track a pane's revision, so a
client can poll for change without re-reading the body.

Commit a624a56 moved raw-tty keys to an early-return branch that knew only
Ctrl-B and bare Escape, and in the same edit deleted the paste branch below
it. That cost Shift-Escape (the unconditional way out of tty mode) and both
paste chords: Ctrl-V and Ctrl-Shift-V reached the child as keystrokes, so an
agent CLI running in a pane took Ctrl-V for its image-paste binding and
answered "No image found in clipboard". Both are restored, with tests.

Nested detection was not subtly broken but deleted: 60367d8 removed
nested.zig's process-ancestry walk and left "am I inside pardes" derived from
PARDES_FORWARD_LOOK, which read "0" both for --nested and for "the listener
did not come up". PARDES_PID now answers that question on its own, checked
with kill(pid, 0); PARDES_9P and PARDES_PANE answer how to reach it; the flag
is gone. The posted-9P registry also self-heals now -- a session that aborts
cannot unlink its own socket, so posting sweeps entries whose target refuses
a connection, symlinks only and on a definite ECONNREFUSED only.

Elsewhere: tty scrolling is sticky-bottom, following new output only from
the last row, with typing and entering raw mode snapping back to live; the
boot layouts are a Boot enum instead of a chain of ifs, and the bare tty
startup (Boot.tty, which main.zig names) opens an empty text pane under the
shell while tests keep Boot.tty_shell; builtins announce themselves on the
message row under a Verbose setting that is on by default; Config prints
each setting the way you would type it back, so WindowOpacity 70 rather than
"WindowOpacity: 70%"; LocationsConfig opens its window only when called bare;
every tagline puts the word that closes the thing last, and a column now
outlives its panes -- closing the last one leaves an empty pane, and only
Delcol, newly on the column tagline, takes the column away.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

1 files changed, 26 insertions, 9 deletions

diff --git a/test/v9fs.py b/test/v9fs.py
index 16916d0b..fa35388f 100644
--- a/test/v9fs.py
+++ b/test/v9fs.py
@@ -43,8 +43,8 @@ def worker(mountpoint, socket, uid, gid, original_namespace):
     assert not any(field.startswith(('shared:', 'master:')) for field in mounted[0].split()[6:])
 
     tree = mountpoint
-    assert {'os', 'index', 'pane', 'new', 'ctl', 'log', 'screen', 'README'} <= set(os.listdir(tree))
-    assert 'self' not in os.listdir(tree)
+    assert {'os', 'index', 'pane', 'status', 'look', 'exec', 'log', 'screen', 'README'} <= set(os.listdir(tree))
+    assert 'self' not in os.listdir(tree) and 'new' not in os.listdir(tree)
     # A direct connection provides independent evidence for VFS reads/writes.
     with Client(socket) as client:
         assert (tree / 'index').read_bytes() == client.read('/index')
@@ -52,14 +52,31 @@ def worker(mountpoint, socket, uid, gid, original_namespace):
 
         before = client.read('/index')
         subprocess.run(['ls', '-l', str(tree), str(tree / 'pane' / '1')], check=True, capture_output=True, timeout=5)
-        subprocess.run(['find', str(tree / 'pane'), str(tree / 'new'), '-ls'], check=True, capture_output=True, timeout=5)
+        subprocess.run(['find', str(tree / 'pane'), '-ls'], check=True, capture_output=True, timeout=5)
         assert (tree / 'README').read_bytes() == client.read('/README')
-        assert (tree / 'new').stat().st_size == 0
         assert client.read('/index') == before, 'browsing created panes'
-        serial = int((tree / 'new').read_bytes().split()[0])
-        another = int((tree / 'new').read_bytes().split()[0])
-        assert serial != another, 'cached factory reused a pane'
-        client.write(f'/pane/{another}/ctl', b'exec Del\n')
+        # mkdir through the kernel mount opens a pane. The editor names it
+        # after its serial, not after the name asked for, so the kernel's own
+        # revalidation of that name may fail; the index is the answer.
+        def serials():
+            return {int(row.split()[0]) for row in (tree / 'index').read_bytes().splitlines()}
+
+        def mkpane(name):
+            known = serials()
+            try:
+                (tree / 'pane' / name).mkdir()
+            except FileNotFoundError:
+                pass
+            made = serials() - known
+            assert len(made) == 1, (name, made)
+            return made.pop()
+
+        serial = mkpane('kernel-made')
+        another = mkpane('kernel-made-again')
+        assert serial != another, 'a second mkdir reused a pane'
+        assert not (tree / 'pane' / 'kernel-made').exists()
+        (tree / 'pane' / str(another)).rmdir()
+        assert another not in serials()
         pane = tree / 'pane' / str(serial)
         wire = f'/pane/{serial}'
         assert str(serial) in os.listdir(tree / 'pane')
@@ -101,7 +118,7 @@ def worker(mountpoint, socket, uid, gid, original_namespace):
         # Reading OS files through the exported tree does not recurse through
         # the mount: the core still lives in the supervisor's namespace.
         assert (mountpoint / 'os' / str(socket.parent).lstrip('/') / 'kernel.txt').read_bytes() == b'initial\n'
-        write_existing(pane / 'ctl', b'exec Del\n')
+        (tree / 'pane' / str(serial)).rmdir()
         assert serial not in [int(row.split()[0]) for row in (tree / 'index').read_bytes().splitlines()]
 
     print('v9fs: namespace isolation, privilege drop, inherited mount, directory refresh, '