Observability improvements

Highlights

This release adds a full observability layer to the ledger: telemetry events
covering command, OCC, transaction, account, and instance lifecycles, plus an
optional trace_context field on commands for distributed tracing
correlation.

Added

• Telemetry instrumentation across the ledger (OBS-1). New
  DoubleEntryLedger.Telemetry module emits 14 point events and one span
  covering command enqueue/claim/retry/dead_letter/idempotency_hit, OCC
  retries, transaction created/posted/archived, account created/updated,
  instance created, and InstanceProcessor start/stop. Metadata uses
  instance_id consistently to keep hot paths DB-free, and emissions are
  wrapped in try/rescue so observational code can never crash business
  logic. Optional telemetry_metrics dependency with dashboard_metrics/0
  for Phoenix LiveDashboard integration. See pages/Telemetry.md for the
  event catalog and integration guide. (2f2a005)
• trace_context field for distributed tracing (OBS-2). Optional
  string-valued map on Command, TransactionCommandMap, and
  AccountCommandMap. Vendor-neutral — carries W3C traceparent/tracestate,
  Datadog trace IDs, or custom correlation IDs — persisted to its own
  column and propagated through Logger metadata via the Traceable
  protocol. Validated as a flat string map with a configurable max key
  count (:max_trace_context_keys, default 10). Ships with v3 migration.
  (3eddb6c)
• Telemetry summary in load tests. LoadTesting.TelemetryCollector
  collects latency samples and lifecycle counters during a load run;
  output now includes P50/P95/P99 latency, OCC retry count, dead letter
  count, and transaction breakdown. (469f213)

Changed

• Dead-letter transitions now log at error level. Intermediate
  states (OCC timeouts, command processing failures) remain at warning
  since they trigger retries; only terminal dead-lettering is elevated.
  Addresses FINDING-OBS-3. (5c6062f)
• mix tidewave alias honors PORT. Defaults to 4000; use
  PORT=4020 mix tidewave when running alongside another app already on
  4000. (3a78550)

Docs

• Expanded custom telemetry handler guidance in pages/Telemetry.md with
  a worked dead-letter alert example covering attach_many,
  Task.Supervisor for blocking I/O, and handler lifecycle gotchas.
  (2a74738)

Migration Notes

Run the v3 migration to add the trace_context column to the commands
table. The column is unindexed — consumers who query by trace context
should add their own index.

Full Changelog: v0.2.0...v0.3.0