feat(telemetry): add Phase 4 consensus tracing with SpanGuard API

Instrument the consensus subsystem with OpenTelemetry spans covering
the full round lifecycle: round start, establish phase, proposal send,
ledger close, position updates, consensus check, accept, validation
send, and mode changes.

Key design choices adapted from the original Phase 4 implementation
to the new SpanGuard factory pattern introduced in Phase 3:

- Add SpanGuard::hashSpan() for category-gated hash-derived trace IDs
  (consensus round spans share trace_id across validators via ledger hash)
- Add SpanGuard::addEvent() overload with key-value attribute pairs
  (used for dispute.resolve events during position updates)
- Add ConsensusSpanNames.h with compile-time span name constants
  following the colocated *SpanNames.h pattern from Phase 3
- Add consensusTraceStrategy config option ("deterministic"/"attribute")
  for cross-node trace correlation strategy selection
- Use SpanGuard::linkedSpan() for follows-from relationships between
  consecutive rounds and cross-thread validation spans
- Use SpanGuard::captureContext() for thread-safe context propagation
  from consensus thread to jtACCEPT worker thread

Spans produced: consensus.round, consensus.proposal.send,
consensus.ledger_close, consensus.establish, consensus.update_positions,
consensus.check, consensus.accept, consensus.accept.apply,
consensus.validation.send, consensus.mode_change

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/telemetry/grafana/provisioning/datasources/tempo.yaml

@@ -8,6 +8,7 @@
 # Phase 1b (infra): Base filters — node identity, service, span name, status.
 # Phase 2 (RPC):    RPC command, status, role filters.
 # Phase 3 (TX):     Transaction hash, local/peer origin, status.
+# Phase 4 (Cons):   Consensus mode, round, ledger sequence, close time.
 
 apiVersion: 1
 
@@ -134,3 +135,34 @@ datasources:
             operator: "="
             scope: span
             type: dynamic
+          # Phase 4: Consensus tracing filters
+          - id: consensus-mode
+            tag: xrpl.consensus.mode
+            operator: "="
+            scope: span
+            type: static
+          - id: consensus-round
+            tag: xrpl.consensus.round
+            operator: "="
+            scope: span
+            type: dynamic
+          - id: consensus-ledger-seq
+            tag: xrpl.consensus.ledger.seq
+            operator: "="
+            scope: span
+            type: static
+          - id: consensus-close-time-correct
+            tag: xrpl.consensus.close_time_correct
+            operator: "="
+            scope: span
+            type: dynamic
+          - id: consensus-state
+            tag: xrpl.consensus.state
+            operator: "="
+            scope: span
+            type: dynamic
+          - id: consensus-close-resolution
+            tag: xrpl.consensus.close_resolution_ms
+            operator: "="
+            scope: span
+            type: dynamic