[AlxcBTW]

Gap 2 of T38. Stacked on #113 (the diff shrinks when that one is merged).

• Stream delay: the host worker stamps each picture with its system clock when it takes it from the ring; the guest's net service picks the host clock's offset from the pings (smallest round trip of the last minute, ClockPicker) and gives it to the viewer (ToWorker::Clock); the viewer works out each picture's age and reports median and 95th percentile of 10 s (DelayWindow).
• Input delay: the guest remembers when it sent each pad sequence number; the host acknowledges the newest applied one with pad_ack (about 4 per second, with how long it held it); the guest takes half the round trip.
• Both figures reach the launcher as stream_delay_ms / input_delay_ms (p50, p95) in the share event, on the guest's PC directly and on the host's PC through the guest's report.
• share_quality (max_kbps, max_height): hub setting, applied to the running share's controller and to later ones.
• Spec and plan updated.

Tests: unit tests for the window, clock picker and frame age (incl. the 32-bit wrap), link and control round trips, command parsing, and two in-process integration tests (figures appear on both sides and are sane on one machine; the host's limit holds the picture at 540p).

Not proven: the figures between two real PCs on two networks (clock offset, asymmetric paths), the engine's copy time (not in the figure by design, see spec).

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