Fix metric wiring gaps audited against live processor output

Several Phase 1 metrics were registered in observability/metrics.ts but
either unwired at the call sites or wired with wrong counts. Production
output showed 11 records ingested per logs but only 4 in metrics. The
fixes below align metric values with actual hot-path activity.

Wiring gaps closed (consumer.ts):
- processor_consumer_reads_total{result=ok|empty|error} — was registered
  but never inc'd. Now fires on each XREADGROUP outcome.
- processor_consumer_records_total — was registered but never inc'd.
  Now fires once per XREADGROUP, with the entry count.

Counts corrected (writer.ts):
- processor_position_writes_total{status} — was inc'd unconditionally
  by 1 per chunk for each of inserted/duplicate. Now inc'd by the
  actual per-status count, and only when count > 0.
- processor_position_writes_total{status='failed'} — was inc'd by 1
  per failed chunk. Now inc'd by chunk.length so every failed record
  is counted.

Counts corrected (main.ts):
- processor_acks_total — was inc'd by 1 per non-empty batch. Now
  inc'd by ackIds.length so every ACK'd ID is counted.

Wiring gap closed (state.ts):
- processor_device_state_evictions_total — internal `evicted` counter
  existed but was never published to metrics. createDeviceStateStore
  now accepts a Metrics injection and inc's on each eviction.

Metrics interface extended (types.ts, metrics.ts):
- Metrics.inc gained an optional third `value` parameter (defaults to 1)
  for batched increments. dispatchInc passes it through to prom-client's
  Counter.inc(labels, value).

Tests updated to reflect the new third arg and the state.ts factory's
new metrics parameter. Total 134 unit tests passing (no count change —
existing tests adjusted, no new tests added; the real verification is
on stage where the metrics are now meaningful again).

src/observability/metrics.ts

@@ -71,8 +71,8 @@ export function createMetrics(): Metrics & {
   collectDefaultMetrics({ register: internal.registry });
 
   const metricsImpl: Metrics & { serializeMetrics: () => Promise<string> } = {
-    inc(name: string, labels?: Record<string, string>): void {
-      dispatchInc(internal, name, labels);
+    inc(name: string, labels?: Record<string, string>, value?: number): void {
+      dispatchInc(internal, name, labels, value);
     },
 
     observe(name: string, value: number, labels?: Record<string, string>): void {
@@ -398,25 +398,27 @@ function dispatchInc(
   r: InternalRegistry,
   name: string,
   labels?: Record<string, string>,
+  value?: number,
 ): void {
+  const v = value ?? 1;
   switch (name) {
     case 'processor_consumer_reads_total':
-      r.consumerReadsTotal.inc(labels ?? {});
+      r.consumerReadsTotal.inc(labels ?? {}, v);
       break;
     case 'processor_consumer_records_total':
-      r.consumerRecordsTotal.inc();
+      r.consumerRecordsTotal.inc(v);
       break;
     case 'processor_decode_errors_total':
-      r.decodeErrorsTotal.inc();
+      r.decodeErrorsTotal.inc(v);
       break;
     case 'processor_position_writes_total':
-      r.positionWritesTotal.inc(labels ?? {});
+      r.positionWritesTotal.inc(labels ?? {}, v);
       break;
     case 'processor_acks_total':
-      r.acksTotal.inc();
+      r.acksTotal.inc(v);
       break;
     case 'processor_device_state_evictions_total':
-      r.deviceStateEvictionsTotal.inc();
+      r.deviceStateEvictionsTotal.inc(v);
       break;
     default:
       // Unknown metric name — silently ignore. This preserves the contract