sl0p: keep a templated tab's purpose, so runtime tabs get a status bar

Clicking the bar's `+` made a tab with no status bar. The button only exposed
it: pressing `n` in TAB mode did the same, in any `sl0ppi up` session.

The chain: a generated layout had no default_tab_template, so a tab created at
runtime had nothing to take a bar from. It had none because
parse_tab_node_with_template read a tab node's name, cwd and focus but never
its `purpose` -- so templating erased the purpose that IS identity here
(`sl0ppi ls` finds project tabs by it), and the CLI worked around that by
inlining the template into every tab instead of emitting it.

Fixed at the root: a templated tab keeps its own purpose, and the CLI emits
`default_tab_template` again. The generated layout also shrinks -- one copy of
the bar's configuration instead of one per tab.

The two properties trade off against each other, so sl0ppi-up-live.py now
asserts both at once: `sl0ppi ls` still sees a tab per project, and every tab
including one created at runtime has exactly one status bar. Confirmed by
reverting each half separately: without the parser fix the tabs lose their
purpose and `ls` shows "alpha —"; without the CLI change the runtime tab has
zero bars, which is the reported bug.

Two measurement traps cost an hour here and are now written down: counting bars
by grepping dump-layout also counts the template and every swap layout ("4 bars
for 3 tabs"), and list-panes stops reporting borderless plugin panes for a tab
once a client has rendered it.

Also in here, from the same session:

- the split button check found buttons by searching for " + " on any row with a
  frame glyph, which matches the stack list's "+N more" once two panes sit side
  by side, and then reports the product broken. It now requires the frame
  corner the button is drawn against.
- the checks killed their sessions but did not delete them, so each run left an
  "EXITED - attach to resurrect" entry: 35 had quietly filled the session
  picker. They now delete-session by name.
- and the trap that cost the user their running client: I killed a bare
  `sl0ppij` because "there are no user sessions on this box". That was a guess,
  not evidence, and wrong. A check's processes always carry that run's own
  socket dir or `attach sl0ptest-...`; a bare `sl0ppij` with a cwd in the repo
  is a human's. Written into the handover next to the `pkill -f` trap, which is
  the same mistake in a different costume.

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