Bring the Go web backend up to parity with the MFC server's screen wall.
A browser tab can now stream several devices over one WebSocket: the
single `watching` field becomes a per-connection watch set guarded by the
hub mutex, capped at 32 devices per socket. The challenge advertises
`caps.multiWatch` and `caps.keyframeRequest`, binary frames stay keyed by
the low 32 bits of the device ID, and connect/disconnect replies echo the
device id and request id so the wall can route concurrent responses.
Input commands (mouse, key, clipboard, rdp_reset) resolve their target
from the watch set: an explicit id must be a current subscription, and an
absent id is accepted only while exactly one device is watched. Legacy
frontends that do not send `multiWatch` keep replace-all semantics, so a
repeated device switch cannot grow the set without bound. Offline
devices are removed from every watch set, and teardown is computed from a
snapshot taken outside the hub lock to avoid re-entrant locking.
Add the `keyframe` command: it replays the cached IDR, then asks the
device to emit a fresh one, rate-limited to one request per device per
second. This exposed a Go-only corruption bug: the client answers the
request with TOKEN_KEYFRAME (134), which the dispatcher treated as a
no-op. That token only appears on forced IDRs because the client's GOP is
effectively infinite, so the dropped IDR left the browser decoding
P-frames against a reference chain it never received, producing
persistent garbling. Relay it as a keyframe instead. Also drop the
cached IDR when the resolution changes, mirroring the C++ server, so a
late joiner cannot receive an old-geometry keyframe.
Add tests for the watch-set transitions and for relaying TOKEN_KEYFRAME.
Co-Authored-By: deepseek-flash
