fix(pty): bound terminal sessions and never block the loop killing one (F-05)

[404SecNotFound]

The remainder of F-05, after PR #43 landed frame validation and a bounded output
queue. Two things were left, and the second is the more serious.

Termination could hang the entire server. _terminate sent SIGTERM and then
called a blocking os.waitpid with no WNOHANG, directly on the asyncio event
loop. A child that ignores SIGTERM froze not just its own tab but every request
the server was serving, with no escalation and no timeout. That is a denial of
service reachable by the terminal's own child process.

terminate() is now a coroutine: SIGTERM, poll with WNOHANG until a grace
period, then SIGKILL, then poll again. Giving up after the second grace leaves a
zombie, which is the right trade: one process table entry against the hang this
exists to remove.

Nothing counted sessions. Every accepted websocket spawned a replicant menu process, so a page reconnecting in a loop, or a few tabs, multiplied real
processes against one host. SessionRegistry caps them globally (4) and per
client (2). The per-client cap is what stops one browser taking every slot and
locking the operator out from another machine.

A refusal returns a sentence, not a bool, and it is written to the terminal
before the socket closes with 1013. A tab that silently fails to open is exactly
the kind of thing that gets reported as "the web UI is broken", which this
session has already demonstrated twice.

This also bounds F-08 in passing: the eps cap is per-process, so capping
processes caps how far it can be multiplied against one collector. It does not
answer F-08, which still needs a decision.

_spawn_command is split out of _spawn so the SIGTERM-ignoring case is
actually testable. It could not be reached at all while the argv was hardcoded,
which is why this defect had no coverage.

The last guard asserts the shape, not just the timing: a timing test passes on a
fast machine even when the blocking call comes back.

839 py. black, ruff, mypy clean. All 11 guards observed to fail first.