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, 18 insertions, 7 deletions

diff --git a/src/ninep/screen.zig b/src/ninep/screen.zig
index 5b68b34c..2684aa41 100644
--- a/src/ninep/screen.zig
+++ b/src/ninep/screen.zig
@@ -83,22 +83,33 @@ pub fn releaseSnapshot(p: *Pardes, req: Req) bool {
     return true;
 }
 
-pub fn readListeners(p: *Pardes, req: Req) Reply {
-    var buf: [512]u8 = undefined;
-    var text = std.Io.Writer.fixed(&buf);
+const listeners_cap = 512;
+
+fn listenersText(p: *Pardes, buf: []u8) []const u8 {
+    var text = std.Io.Writer.fixed(buf);
     if (p.fs.socket_path.len != 0)
-        text.print("unix!{s}\n", .{p.fs.socket_path}) catch return Reply.fail(req.tag, E.IO);
+        text.print("unix!{s}\n", .{p.fs.socket_path}) catch return text.buffered();
     for ([_]?std.Io.net.IpAddress{ p.fs.tcp_address, p.fs.quic_address }, [_][]const u8{ "tcp", "quic" }) |maybe, transport| {
         const address = maybe orelse continue;
         switch (address) {
             .ip4 => |ip| text.print("{s}!{d}.{d}.{d}.{d}!{d}\n", .{ transport, ip.bytes[0], ip.bytes[1], ip.bytes[2], ip.bytes[3], ip.port }) catch
-                return Reply.fail(req.tag, E.IO),
+                return text.buffered(),
             .ip6 => |ip| text.print("{s}!{f}!{d}\n", .{ transport, std.Io.net.Ip6Address.Unresolved{ .bytes = ip.bytes, .interface_name = null }, ip.port }) catch
-                return Reply.fail(req.tag, E.IO),
+                return text.buffered(),
         }
     }
+    return text.buffered();
+}
+
+pub fn listenersLen(p: *Pardes) u64 {
+    var buf: [listeners_cap]u8 = undefined;
+    return listenersText(p, &buf).len;
+}
+
+pub fn readListeners(p: *Pardes, req: Req) Reply {
+    var buf: [listeners_cap]u8 = undefined;
     const out = p.fs.stage(p.gpa);
-    out.appendSlice(p.gpa, text.buffered()) catch return Reply.fail(req.tag, E.NOMEM);
+    out.appendSlice(p.gpa, listenersText(p, &buf)) catch return Reply.fail(req.tag, E.NOMEM);
     return tree.stagedReply(p, req);
 }