Make pty/run and /log streams, and take refused lines back cleanly

Review fixes to pty/run and /log:

- Both are streams with a per-open cursor. A shell's exec 3<>file shares
  one offset between write and read, so cat <&3 after echo make >&3 asked
  for offset 5 and got "0" instead of "exit 0"; log after follow lost its
  first bytes the same way.
- A run is accepted only in pardes's own tagged input phase, so a nested
  shell's prompt (ssh, a shell with its own integration) is never taken for
  this shell's.
- A line the shell refused is taken back so the next finds the prompt
  clear: bash's continuation prompt (now tagged k=c) gets Ctrl-C and the
  answer waits for the fresh prompt; a bash syntax error (no C, but a D) is
  answered at once; fish's kept line gets Ctrl-U (a Ctrl-C sent while fish
  redraws is lost). fish's right prompt no longer reads as typed input.
- bash marks a command's start from PS0 on bash 4.4+, not a DEBUG trap, so
  a user's own DEBUG trap (bash-preexec, atuin) survives. bash's own job
  notices now show as in plain bash (ttyfollow golden).
- 32 run slots; a new line on an open starts a fresh answer.

Checked live in bash, fish, and fish with a right prompt.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 5 insertions, 2 deletions

diff --git a/src/ninep/events.zig b/src/ninep/events.zig
index fa0a4975..20cdeda4 100644
--- a/src/ninep/events.zig
+++ b/src/ninep/events.zig
@@ -208,10 +208,11 @@ pub fn readLog(p: *Pardes, req: Req) Reply {
     if (slot.node != req.node) return Reply.fail(req.tag, E.INVAL);
     const frozen = slot.bytes orelse &.{};
     const out = p.fs.stage(p.gpa);
-    if (!slot.follow or req.off < frozen.len) {
-        const off = @min(req.off, frozen.len);
+    if (!slot.follow or slot.pos < frozen.len) {
+        const off = @min(slot.pos, frozen.len);
         const len = @min(frozen.len - off, req.size);
         out.appendSlice(p.gpa, frozen[off..][0..len]) catch return Reply.fail(req.tag, E.NOMEM);
+        slot.pos += len;
         return .{ .tag = req.tag, .payload = .{ .staged = @intCast(len) } };
     }
     const q = &p.fs.log;
@@ -678,6 +679,8 @@ test "the log records whether or not anyone reads, and an open that follows wait
     const frozen = call(p, .{ .tag = 6, .op = .read, .node = log, .handle = fh, .size = 8192 }).bytes.len;
     try testing.expectEqual(Status.err, call(p, .{ .tag = 7, .op = .write, .node = log, .handle = fh, .data = "tail\n" }).reply.status);
     try testing.expectEqual(Status.ok, call(p, .{ .tag = 8, .op = .write, .node = log, .handle = fh, .data = "follow\n" }).reply.status);
+    // A stream: after `echo follow >&3` a shell reads at offset 7, and gets
+    // what comes next, not the frozen text from byte 7.
     const rdf = struct {
         fn next(pp: *Pardes, node: u64, h: u32, off: u64) th.Answer {
             return call(pp, .{ .tag = 9, .op = .read, .node = node, .handle = h, .off = off, .size = 8192 });