feat(node): the stream capacity is the operator's to set, and a log that says who stopped

Bounding the client's read-ahead changed what a transcode slot is. It used to
be a burst — the browser took segments as fast as it could append them, so a
slot came back within the minute whatever the length of the film. Now it is
held for as long as someone is watching, so the cap counts simultaneous
viewers, and two of them meant the third was refused for the next hour and a
half.

The right number depends on the machine, so it belongs to the operator:
`[node] max_concurrent_streams` in node.toml, or
MESHBAY_MAX_CONCURRENT_STREAMS. Default 8 — one ffmpeg per viewer, remuxing
rather than encoding, idle on a pipe for most of the film. Zero, a negative
number, a non-number and a bool are refused with a warning naming the setting:
`Semaphore(0)` is not "no limit", it is a node where no video ever plays and
nothing says why, and TOML `true` would have become 1 by way of `int()`.

A stream also ends on the peer's silence now rather than on its stinginess. A
viewer buffered well ahead deliberately grants nothing for minutes, and the
old budget accumulated over the whole wait, so a keepalive that granted no
credit could not keep a paused film alive.

The rest is diagnosis, which is what this cost. `client_diag` carries the
player's own view — readyState, refused appends, buffered ranges, the video
element's error — into the node's log at DEBUG, next to the node's view of the
same stream. It is the only window into a phone, and every field is
stringified and cut short because all of it is peer-controlled. The node also
logs the first keepalive, which distinguishes a paced client from an unpaced
one at a glance, and progress every hundred segments, whose last line says
where a stream stopped and which side stopped it.

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