fix(tools): Use ␞ (U+241E) as snapshot_pane delimiter, matching libtmux's FORMAT_SEPARATOR

why: The earlier delimiter fix picked `\t` on the theory that tmux
always passes tabs through verbatim. That's true but incomplete:
tabs are also legal (if rare) in Linux paths, so a pane_current_path
containing a tab would still silently shift the parsed fields. And
while researching which tmux versions escape ASCII control chars
like 0x1f, the clean answer emerged from libtmux itself: since
commit f88d28f2 (Jan 2023) libtmux has used the printable Unicode
glyph `␞` (U+241E, "SYMBOL FOR RECORD SEPARATOR") as FORMAT_SEPARATOR
for exactly this class of parsing.

Two independent reasons ␞ is strictly safer than `\t`:

1. tmux's utf8_strvis (utf8.c:663-675) explicitly copies valid
   UTF-8 multi-byte sequences verbatim, bypassing the vis()
   escape that turns ASCII control chars into octal strings. `␞`
   is a valid 3-byte UTF-8 sequence (0xe2 0x90 0x9e), so no tmux
   version — stable, alpha, or master — can escape it through the
   control-char path that broke `\x1f` on CI.

2. `␞` is a printable Unicode symbol that realistically cannot
   appear in a pane title, a running command name, or a
   filesystem path. That gives us immunity to the tab-in-path
   vulnerability that `\t` doesn't have.

This is the same delimiter libtmux uses internally for every
object's format parsing, tested across every tmux version in the
libtmux CI matrix for multiple years. We gain the same coverage
for free.

what:
- Switch _SEP in snapshot_pane from "\t" to "␞".
- Update the inline comment to cite utf8_strvis + libtmux's
  FORMAT_SEPARATOR as the rationale.
- Update the monkeypatched fake_cmd in
  test_snapshot_pane_pads_short_display_message_output to split /
  rejoin on "␞".
- Defensive padding retained unchanged.

This supersedes the `\t` choice from the immediately-prior fix
commit; that commit's defensive-padding contribution remains the
load-bearing anti-IndexError guarantee. Verified locally against
tmux 3.6a. Expected to clear the CI matrix failures on 3.2a, 3.3a,
3.4, 3.5, 3.6 stable, and master.

Author: tony

src/libtmux_mcp/tools/pane_tools.py

@@ -670,17 +670,17 @@ def snapshot_pane(
         window_id=window_id,
     )
 
-    # Fetch all metadata in a single display-message call. Use a tab as
-    # the delimiter: tmux passes tabs through verbatim in
-    # display-message output, whereas other ASCII control characters
-    # (e.g. 0x1f / Unit Separator) get C-escaped to literal "\037"
-    # strings on tmux >=3.2 / <3.6-rc, which corrupts parsing. Tabs in
-    # pane_title are silently rejected by tmux's `select-pane -T`
-    # input sanitizer, so the `\t` delimiter is safe against that
-    # vector. Tabs in pane_current_path are legal on Linux but
-    # vanishingly rare; the defensive padding below limits the blast
-    # radius to a single truncated field rather than an IndexError.
-    _SEP = "\t"
+    # Fetch all metadata in a single display-message call. Use the
+    # printable Unicode glyph ␞ (U+241E, "SYMBOL FOR RECORD SEPARATOR")
+    # as the delimiter — the same choice libtmux itself uses for
+    # FORMAT_SEPARATOR. tmux's utf8_strvis (tmux/utf8.c) copies any
+    # valid UTF-8 multi-byte sequence verbatim, bypassing the vis()
+    # escape that turns ASCII control chars like 0x1f into literal
+    # "\037" in display-message output on some tmux builds. And ␞ is
+    # safe against the false-positive path that a tab delimiter has:
+    # tabs are legal (if rare) in Linux paths and could realistically
+    # appear in pane_current_path.
+    _SEP = "␞"
     fmt = _SEP.join(
         [
             "#{cursor_x}",

tests/test_pane_tools.py

@@ -584,8 +584,8 @@ def fake_cmd(self, cmd_name, *args, **kwargs):  # type: ignore[no-untyped-def]
             # simulate an old tmux that dropped several unknown format
             # variables. Without defensive padding, parts[2..10] would
             # IndexError.
-            parts = result.stdout[0].split("\t") if result.stdout else [""]
-            result.stdout = ["\t".join(parts[:2])]
+            parts = result.stdout[0].split("␞") if result.stdout else [""]
+            result.stdout = ["␞".join(parts[:2])]
         return result
 
     monkeypatch.setattr(mcp_pane.__class__, "cmd", fake_cmd)