[chain-pr 4/12] Add deterministic sampling helpers and shared session/telemetry context

[johnHackworth]

This PR is part of a chain split from #0 v7 by chain-pr.

Changes

Move isSampled / sampleUsingKnuthFactor / correctedChildSampleRate from rum-core into core. Add startTelemetrySessionContext shared hook in core/contexts. Update userContext to drop AssembleTelemetry registration. Add sampling test fixtures (LOW_HASH_UUID, HIGH_HASH_UUID, MID_HASH_UUID).

Refactoring rationale

Moves the deterministic sampling primitives from rum-core to core so both RUM and Logs (and the developer extension) can apply consistent sampling decisions. Introduces a shared telemetry session context hook, replacing scattered AssembleTelemetry registrations. This is a structural prerequisite for the unified session manager that follows.

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Draft — pending author review. Merge in order unless marked parallel.

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