OpenTelemetry sink over the StoreTelemetry seam

[matt-ramotar]

Ships the org.mobilenativefoundation.store:opentelemetry experimental artifact — an OpenTelemetry sink over the StoreTelemetry seam — per the approved design and implementation plan:

• Design: docs/store6/designs/opentelemetry-sink-design.md
• Plan: docs/store6/designs/opentelemetry-sink-implementation-plan.md

What ships

• New KMP module :opentelemetry (androidTarget + jvm via the subset convention plugin, created jvmAndroidMain intermediate source set), publishing opentelemetry / opentelemetry-jvm / opentelemetry-android at 6.0.0-SNAPSHOT.
• One public class OpenTelemetryStoreTelemetry mapping the six telemetry hooks to the five v0 instruments plus an opt-in synthesized store6.fetch span. Every public declaration is @ExperimentalStoreApi; hooks never throw after construction; StoreError messages/causes are never exported.
• 19 jvmTest tests: metric vocabulary, all six error.type literals + no-diagnostics privacy sweep, span synthesis edges, three hostile-SDK robustness fixtures, concurrency, instrumentation-scope version drift guard, and two real-engine integration tests.
• Runnable JVM sample :opentelemetry-sample that machine-checks its exported telemetry and exits on its own.
• CI wiring in store6.yml: build + sample-run steps, core-internal-access and TD-8 gate coverage, and the klib-publication-check entries (two coupled edits).
• opentelemetry/README.md + opentelemetry/SIGNALS.md (vocabulary v0), and the STABILITY.md §3 artifact row.

Descoped steps

None. All plan tasks 0–11 completed in full; the one [descope-allowed] test (aThrowingSetStatusStillEndsTheSpan) is implemented and green.

Execution notes (deviations from the plan text, each verified green)

1. Steps 1.7/2.2's combined :opentelemetry:build :opentelemetry:apiDump single invocation trips Gradle 8.11's implicit-dependency validation (klibApiExtractForValidation reads the file klibApiDump writes when co-scheduled). The two goals run as separate sequential invocations; no file changes.
2. Two map-equality assertions (metrics overflow test, concurrency test) need explicit mapOf<String?, Long>(…) type arguments — Kotlin cannot unify Map<String, V> with Map<String?, V> (invariant key parameter).
3. The two robustness tracer fixtures additionally override TracerProvider.tracerBuilder: the interface's default implementation returns a no-op builder, which silently bypassed wrappers that override only get(...) (the sink acquires its tracer through tracerBuilder). Without the fix, the setStatus test fails and the throwing-tracer test passes vacuously.
4. SpanBuilder.setAttribute override signature is value: T? per the compiler, as the plan's worker notes pre-authorize.

docs-sync

STABILITY.md is listed in .github/docs-sync-sources.txt, so this PR requires the docs-sync-ack label (applied) for the docs-sync-guard job.