Add vendor-neutral telemetry (metrics + tracing) for v6.4.0

[jchristn]

Summary

Adds standardized, vendor-neutral telemetry to WatsonTcp so it can be observed by Radiant, the OpenTelemetry SDK, Prometheus, or any compatible host with no dependency on any telemetry
backend. WatsonTcp emits into a System.Diagnostics.Metrics.Meter and a System.Diagnostics.ActivitySource, both named WatsonTcp; hosts subscribe by name. This is a minor release (6.3.2 →
6.4.0), purely additive, no breaking changes.

What's included

• WatsonTcpMetrics (public) — the consumer contract: stable source names, 24 metric names, 6 span names, UCUM units, and tag keys as constants.
• WatsonTcpInstrumentation (internal, IDisposable) — per-instance Meter + ActivitySource, all counters/histograms/observable-gauges and span factories; disposed with its owner. Every recording
  call is null-guarded and fire-and-forget, so telemetry can never throw into the send/receive/connection path.
• Wired through the client/server lifecycle: send/receive + bytes, connection admission outcomes, disconnections by DisconnectReason, handshake, authentication, connection authorization, synchronous
  request/response/timeout/expiry, exceptions (funnelled through one helper), transient accept errors, stream drain, and uptime.
• Metric tags are low-cardinality only (role, protocol, outcome, reason, …); high-cardinality identifiers (client GUID, remote endpoint, conversation GUID) go on spans only.
• Settings.EnableMetrics / Settings.EnableTracing (default true) on both settings classes; when metrics are off, no Meter is created at all.
• References System.Diagnostics.DiagnosticSource 8.0.1 on the down-level target frameworks only (in-box on net8.0/net10.0).

Testing

• 17 new positive and negative telemetry tests using the BCL MeterListener/ActivityListener (zero third-party test deps), plus a new telemetry suite and a --suite filter for the console runner.
• Covers counters/bytes, connection gauges + outcomes, disconnect-by-reason, sync round-trip + timeout, handshake success + failure, auth success + failure, authorization allow + reject, blocked-IP
  rejection, transient accept error, metrics-disabled → no measurements, tracing-disabled → no spans, span tags, and a cardinality guard.
• 126/126 tests pass on net8.0 and net10.0 (109 existing + 17 new), no regressions.
• Release build clean (0 warnings) across all six target frameworks.

Docs

README (New in v6.4.0), CHANGELOG, ARCHITECTURE (§11 Telemetry), CLAUDE.md, and a new consumer-facing TELEMETRY.md (full metric catalog, tag dictionaries, Prometheus series names, PromQL examples). The
design/implementation plan is preserved at archive/TELEMETRY_PLAN.md.