fix(realtime): propagate observability telemetry options

[AdirAmsalem]

Summary

• Propagate the SDK telemetryEnabled option through the realtime subscribe path instead of forcing telemetry off.
• Require injected RealtimeObservability in WebRTCManager / WebRTCConnection, removing the hard-coded disabled observability fallback with an empty API key.
• Rename inter-frame delay Variance stat to StdDev to match the actual computation.

Checks

• pnpm --filter @decartai/sdk typecheck
• pnpm --filter @decartai/sdk format:check (passes with existing warnings/info)
• pnpm --filter @decartai/sdk test -- --run
• pnpm --filter @decartai/sdk build (passes; existing tsdown warning about define option)
• Independent Claude Code review of diff vs origin/alon/livekit-pr1-observability

Base for #132.

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Note

Medium Risk
Changes realtime telemetry behavior (subscribe path now honors telemetryEnabled) and makes WebRTCManager/WebRTCConnection require an injected RealtimeObservability, which can be a breaking change for any consumers constructing these classes directly. Renaming a WebRTCStats field also risks downstream type/runtime expectations.

Overview
Realtime subscribe sessions now honor the SDK’s telemetryEnabled option and immediately call observability.sessionStarted(sid) so telemetry/diagnostics are associated with the session.

WebRTCManager and WebRTCConnection no longer create an internal fallback RealtimeObservability (previously hard-coded telemetry off with an empty API key); callers must pass observability explicitly, and unit tests are updated accordingly.

In WebRTCStats, the inter-frame delay metric is renamed from interFrameDelayVarianceMs to interFrameDelayStdDevMs to match the actual std-dev calculation.

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