acmefs: a pane's terminal gets pty/data, pty/ctl and pty/status

Step 3 of the 9P chain (docs/9p.typ 12.3, docs/registry.typ 9P-8). Nothing here
is about 9P: it lands in the FUSE-served tree and any later transport inherits it.

A script could write into a terminal that already existed and read its rendered
scrollback. It could not START one, RESIZE one or SIGNAL one. Two of those were
already effects the core emits, so `exec` and `winsize` are existing
capabilities acquiring a name; only `sig` is new, and it brings the one new
host method, `push_pty_signal`.

  pty/ctl     winsize <cols> <rows> | sig INT|TERM|HUP|QUIT|KILL | exec
              one verb per line, validate-all then apply-all, EINVAL applies
              nothing -- `writeCtl`'s shape and `writeCtl`'s reason
  pty/status  cols, rows, tty-taken as three %11d fields
  pty/data    write is input to the process; read is the RAW output stream,
              gated on a reader count so a pane nobody reads costs one branch

A pane that is not a terminal has no pty/ at all: the lookup is ENOENT and
readdir does not list it.

`PaneFile` is an enum(u4) and this takes it from 11 values to 15. ONE REMAINS.
That is also why pty/ is a DIRECTORY and not three more flat names -- a
subdirectory costs one value and buys its own namespace, so `ctl` and `data`
did not have to be renamed.

Two things the core does not know, and which are therefore not invented: a
child's EXIT STATUS (a shell's death is `Event.eof`, which removes the pane,
so there is no directory left to read it in) and RAW/COOKED (the core never
sets a termios; the mode belongs to the program on the far side).

Verified live against a daemon: pty/ appears only on the terminal pane; a
`winsize 0 24` and a `sig SIGINT` are refused; a bad verb beside a good one
applies neither; `echo pty-works` written to pty/data runs in the shell and its
output reaches the body; and a blocking read of pty/data returns the raw stream,
OSC 133 marks and all. fs-bench unchanged and still zero allocations.

2 files changed, 28 insertions, 13 deletions

diff --git a/src/detached/server.zig b/src/detached/server.zig
index 9a8dc24f..d8373149 100644
--- a/src/detached/server.zig
+++ b/src/detached/server.zig
@@ -590,14 +590,14 @@ pub const Session = struct {
         return @ptrCast(@alignCast(ctx.?));
     }
 
-    /// Seventeen methods, and NOT the fullest host in the tree — that claim
+    /// Eighteen methods, and NOT the fullest host in the tree — that claim
     /// stood here, was believed, and was copied into docs/detached.md before an
-    /// audit counted the others. The tty and SDL shells fill NINETEEN each
-    /// (everything but `pull_gpio_toggle` and `push_detach`) and macOS fourteen,
+    /// audit counted the others. The tty and SDL shells fill TWENTY each
+    /// (everything but `pull_gpio_toggle` and `push_detach`) and macOS fifteen,
     /// so this host is the only one that implements `push_detach` and otherwise
     /// the least complete of the three desktop hosts. What is true is narrower
     /// and is the point anyway: it performs every MACHINE-LOCAL effect there is,
-    /// and the four of host.zig's twenty-one it leaves null are null because
+    /// and the four of host.zig's twenty-two it leaves null are null because
     /// there is nothing here for them to do. Two of those four are real losses a
     /// person can notice — no `pull_lsp` and no `pull_pipe`, because both want
     /// the worker pool this deliberately single-threaded loop does not have. The
@@ -619,6 +619,7 @@ pub const Session = struct {
         .push_spawn = spawn,
         .push_pty_write = ptyWrite,
         .push_pty_resize = ptyResize,
+        .push_pty_signal = ptySignal,
         .pull_tty_taken = ttyTaken,
         .push_write_file = writeFile,
         .push_write_dump = writeDump,
@@ -771,6 +772,18 @@ pub const Session = struct {
         _ = posix.system.ioctl(fd, TIOCSWINSZ, @intFromPtr(&ws));
     }
 
+    /// `pty/ctl`'s `sig` — and the host where it matters most, because these
+    /// shells outlive every frontend: a script that signals a build in a
+    /// detached session is signalling a process nobody has a terminal on.
+    /// `fd < 0` is a pane with no shell, the same silence `ptyWrite` gives it.
+    fn ptySignal(ctx: ?*anyopaque, pane: u8, sig: pardes.PtySignal) void {
+        const s = of(ctx);
+        if (pane >= s.ptys.len) return;
+        const pt = s.ptys[pane];
+        if (pt.fd < 0) return;
+        look.signalTty(pt.pid, pt.fd, sig);
+    }
+
     /// Is this pane's tty still the prompt we forked, or has a program taken it?
     ///
     /// Answerable at all only because the pty is HERE. While a pane's shell
diff --git a/src/detached/wire.zig b/src/detached/wire.zig
index a84c2a68..23a85158 100644
--- a/src/detached/wire.zig
+++ b/src/detached/wire.zig
@@ -39,16 +39,18 @@
 //! frontend must not have to have been built with the core's options — so it is
 //! ALWAYS on the wire and dropped on arrival by a build with nowhere to put it.
 //!
-//! WHAT IS NOT HERE. The seam has twenty-one methods; this carries FIVE of
+//! WHAT IS NOT HERE. The seam has twenty-two methods; this carries FIVE of
 //! them — `push_present` as `frame`, `push_set_clipboard`,
-//! `pull_read_clipboard`, `push_open_link` and `push_detach` — and the sixteen
-//! it does not are named here with their reasons. The five are spelled out
-//! because this arithmetic has now gone stale twice in one day, once when the
-//! machine-local eight moved into the daemon and once when `detach` arrived,
-//! and a count nobody can check against a list is a comment that rots quietly.
-//!   * The eight machine-local ones — `push_spawn`, `push_pty_write`,
-//!     `push_pty_resize`, `push_write_file`, `push_write_dump`,
-//!     `push_watch_file`, `push_watch_theme`, `push_dump_themes` — are
+//! `pull_read_clipboard`, `push_open_link` and `push_detach` — and the
+//! seventeen it does not are named here with their reasons. The five are
+//! spelled out because this arithmetic has now gone stale twice in one day,
+//! once when the machine-local eight moved into the daemon and once when
+//! `detach` arrived, and a count nobody can check against a list is a comment
+//! that rots quietly.
+//!   * The nine machine-local ones — `push_spawn`, `push_pty_write`,
+//!     `push_pty_resize`, `push_pty_signal`, `push_write_file`,
+//!     `push_write_dump`, `push_watch_file`, `push_watch_theme`,
+//!     `push_dump_themes` — are
 //!     performed by the detached core ITSELF, through `host_io.zig`. A unix
 //!     socket means it is on the same machine, so there is no question of
 //!     whose disk or whose process table is meant, and a pane's shell has to