[chain-pr 3/12] Replace BoundedBuffer with BufferedObservable and add onDrop

[johnHackworth]

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

Changes

Remove the deprecated BoundedBuffer abstraction in favor of BufferedObservable, adding an onDrop callback for telemetry. Update bufferContextCalls and core index exports accordingly.

Refactoring rationale

Consolidates two overlapping buffering abstractions into one. BoundedBuffer is removed entirely and bufferContextCalls is reimplemented on top of BufferedObservable. The new onDrop callback enables tracking of dropped data via telemetry, which downstream consumers (preStartRum/preStartLogs) rely on.

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

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