OpenTelemetry Activity emission from gateway servers (HiSlip/VXI-11/SOCKET)

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Context

ADR 0040
ships OpenTelemetry exporter wiring in v0.1.0 (Activity + Meter,
OTLP via [telemetry]), but the gateway servers
(`IviCli.Server.HiSlip`, `IviCli.Server.Vxi11`,
`IviCli.Server.Socket`) reserve an ActivitySource name but
never emit Activities.

CHANGELOG.md → Known limitations:

Gateway-server OpenTelemetry Activity emission deferred
(ADR 0040 §Out-of-scope) — Activity source reserved but unused.

This means dashboards / traces show the CLI side of a query/write
but not the gateway side of the same call when the user is
issuing SCPI through a gateway to a backend.

Scope

• Add Activity start/stop around per-session lifecycle and
  per-message handling in each gateway server.
• Propagate W3C trace context across the HiSlip handshake when
  the client sends traceparent (HiSlip is binary; piggyback in
  a vendor-specific PDU or via OTLP-context-injection mode).
• Same for VXI-11 (less standard but should still emit local
  Activities even without propagation).
• Document the new spans in ADR 0040 (update in place).

Acceptance

• ivicli api start + ivicli server start hislip-srv produces
  end-to-end traces from HTTP request → gateway → backend →
  device, viewable in Jaeger / Tempo.
• New spans documented in ADR 0040 §Decision.