sl0ptty: M3 — tabs, purposes, and the JSON control API

Tabs are layout trees, one per tab, and panes in every tab keep running: fds,
pumping and reaping span all of them, only the current one is composed. A
one-row strip shows them (M5 makes it the real status bar) and each label
registers a hit, so clicking a tab switches to it through the same mechanism
as every other click.

  C-a c  new tab      C-a n / C-a p  cycle      C-a 1..9  select

Purposes on panes and tabs with D8's trust model implemented as a lock:
declared (from a layout or an operator) outranks in-band, cannot be overridden,
and everything is sanitised to [A-Za-z0-9_.:/-] on ingest — 'project:foo
bar/../$(rm -rf)' becomes 'project:foobar/../rm-rf'. The in-band path arrives
with the OSC scanner in M4 and always passes declared:false; the refusal is
already tested.

The control API is now JSON in, JSON out:

  {"cmd":"new-tab","name":"agent","purpose":"project:x.deadbeef"}
  {"ok":true,"id":2}

with panes/tabs/snapshot/send/raw/resize/split/focus/close/select-tab/
set-name/set-purpose/alive/quit. Panes are addressed by id, not by 'the
focused one', so a background tab can be scripted. The bare-verb form stays as
a harness alias and runs the same code, so the tests cannot drift from the API.
Adds a small JSON reader (jsonval.c) — we could only emit until now.

129 checks green.

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