retire _transportable's refusal of -X=value, -t... and -- once a psmux release carries the psmux/psmux#795 fix #914

psmux fixed psmux/psmux#795 on master in 20ef00fb. Until then, its client argv normalizers rewrote option values shaped like flags: -x=foo was stored as -x foo, and -t… and a bare -- were refused. bmad-loop works around that in #908 (#905): _transportable refuses those three shapes, so they take the visible path (a warning, and the key freed) instead of being stored corrupted. As of 2026-10-14 no release carries the fix. The latest tag is v3.4.0, and git tag --contains 20ef00fb is empty.

What retires: the clause in PsmuxMultiplexer._transportable (src/bmad_loop/adapters/psmux_backend.py):

if value == "--" or value.startswith("-t") or re.match(r"-[A-Za-z]=.", value):
    return False

Also retire the sentence in docs/multiplexer-backends.md ("Refused option values") that names these three shapes.

What stays: every other _transportable rule (empty, a double quote, a line break, edge whitespace). Each one has its own reason in the docstring, and none of them depends on psmux/psmux#795.

The trigger

tests/test_psmux_live.py::test_premise_an_argv_flag_shaped_option_value_is_not_stored pins the 3.4.0 behaviour for each shape: -x=foo reads back -x foo at rc 0, and -t… and -- fail. It turns red on a psmux build that stores them verbatim. The weekly psmux-canary workflow runs the live gate against the latest psmux release, so that is where the red shows first.

How to retire it (not a plain deletion)

Retire it behind the version gate, as #900 did for psmux/psmux#583:

1. Move _LAST_UNSUPPORTED to the last release without 20ef00fb (the gate is exclusive), and move the CI gate's pin (ci.yml, version:) to the first release that has it.
2. Delete the clause and its doc sentence.
3. Turn the premise row into an ordinary-direction test_adopted_* row: the three shapes are stored verbatim. Show that it fails against a fake of the 3.4.0 behaviour.
4. Add a CHANGELOG entry: an upgrade item for the floor, plus Removed for the refusal.

On any admitted build below the fix, removing the clause brings back silent corruption of a -X=value option value. A tag that reads back differently makes a window silently unprunable.

Checked: bmad-loop is not exposed to the rest of psmux/psmux#795

While fixing it, the maintainer found the same corruption in rename-session, set-buffer, new-window -n and display -p. bmad-loop never passes those commands a value starting with -:

• session names are bmad-loop-<run id> and bmad-loop-ctl;
• window names are run, sweep and control-window names that start with a letter;
• it does not use set-buffer or rename-session;
• its display-message -p calls pass only format strings that start with #{.

So _transportable remains the only workaround to retire.

Refs #905, #908