feat(telemetry): correlate tx spans to the ledger being worked on

Add shared current_ledger_seq / current_ledger_hash span attributes so a
transaction's work can be joined to the ledger trace that produced it, and
fix discrepancy D1 (txq.enqueue was a detached trace root).

- Define current_ledger_seq / current_ledger_hash once in SpanNames.h and
  re-export via `using` from TxQ/TxApply/Tx span-name headers. These name the
  ledger being worked on (open/tentative apply or in-flight consensus build),
  distinct from ledger_seq (the built/validated ledger on ledger.build /
  consensus.round). Named after the RPC field ledger_current_index.
- txq.enqueue: set current_ledger_seq/hash from the view, and parent the span
  to the caller's tx.process span via an explicit captured SpanContext (new
  trailing TxQ::apply param) instead of a detached root. The parent is
  explicit, not ambient-inherited, and the ScopedSpanGuard scope is RAII-bound
  to the synchronous apply, so it cannot leak onto a reused worker (D1 fix).
  On the open-ledger rebuild path no tx.process context exists, so it stays a
  root and the attribute provides the correlation.
- tx.preclaim / tx.transactor: set both attributes from their ledger view.
  tx.preflight is stateless (no view) and is the documented exception.
- tx.process / tx.receive: set current_ledger_seq from the current open ledger
  index at submit/receive time (no hash: not yet applied to a ledger).
- Contract test pins the two new attribute key strings.

Neither key is a spanmetrics dimension, so there is no metric-cardinality
impact. Dashboards/collector/docs land on the later phases per the chain split.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/tests/libxrpl/telemetry/TxApplySpanNames.cpp

@@ -51,3 +51,12 @@ TEST(TxApplySpanNames, stage_values_are_the_three_pipeline_stages)
     EXPECT_EQ(std::string_view(tx_apply_span::val::preclaim), "preclaim");
     EXPECT_EQ(std::string_view(tx_apply_span::val::apply), "apply");
 }
+
+TEST(TxApplySpanNames, current_ledger_keys_are_shared_constants)
+{
+    // "ledger being worked on" keys, re-exported from the base SpanNames.h so
+    // every emitter (tx.*, txq.*, apply pipeline) shares one string. Set on
+    // preclaim/transactor (view-bearing); preflight omits them (stateless).
+    EXPECT_EQ(std::string_view(tx_apply_span::attr::currentLedgerSeq), "current_ledger_seq");
+    EXPECT_EQ(std::string_view(tx_apply_span::attr::currentLedgerHash), "current_ledger_hash");
+}