hqsh 0.2.0: shared attach, like tmux (--steal, --read-only)

[ralyodio]

Several clients can attach to one session at once, tmux style.

Behaviour

• Shared by default: every attached client gets all output; any client's input goes to the PTY.
• --steal / -d (tmux attach -d): the others get a new DETACHED frame (type 11) and exit with "detached by another client" instead of reconnecting. Sent on the first connection only; reconnects never steal.
• --read-only / -r: the daemon ignores this client's INPUT (and the client does not send keys). Ctrl-^ . still detaches.
• Size: smallest cols and smallest rows among attached clients (read-only included), recomputed on attach/detach/RESIZE.
• Resume: per client, from the shared ring, as before.
• ACK: the ring is trimmed only to the minimum ACK across attached clients; the 4 MiB budget still caps it.
• Backpressure: each client has its own writer and cursor into the ring. The PTY is paced to the most caught-up client (2 MiB window): one client = plain backpressure as in 0.1 (no drop-and-reconnect loop on a slow link); with several, a lagging or stalled client never holds the others, and is dropped once it falls off the ring or blocks a write for 5 s, then resumes via the gap path.
• server list --json: keeps attached (bool), adds clients (count). Text: attached (N clients) when N > 1.
• Silent: nothing is printed when others attach.

Wire compatibility

• The HELLO flags byte sits after a second 0 byte; with no flags the HELLO is byte-identical to 0.1, so the shared default costs nothing.
• WELCOME flag bit1 = daemon does shared attach. STATUS appends a u16 client count (0.1 daemons: 1/0 inferred).
• 0.1 client on a 0.2 daemon: a shared read-write client (after a steal it reconnects, since it does not know DETACHED).
• 0.2 client on a 0.1 daemon: works (verified), with 0.1 take-over semantics; end the session and re-attach to get shared attach.

Tests

• Unit: minSize, minAck, ACK-min trim against the ring, mustWait pacing, ring Behind/First, HELLO flags and STATUS encodings.
• Integration (real /bin/sh over pipes): two clients see each other's input; stty size follows the min rule through resize and detach; read-only input is ignored; --steal sends DETACHED and drops the others; a stalled client is dropped while the other receives all 6 MB contiguous; a real client.Run pair plus a stealer (the stolen ones exit 0 with one dial each).
• E2E on dev2 over ssh localhost: 2 clients + a read-only one + a stealer; a 0.2 client on a running 0.1 daemon; a 0.1 client alongside a 0.2 client on a 0.2 daemon.

Also

• Release workflow: a concurrency group per tag. v0.1.0 got two push events for one tag 2 s apart (the workflow has a single trigger), so two runs raced to create the release.
• CI: go test -race on Linux.

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