Add delta encoder for timeseries compression

[barboraplasovska]

What and why?

Adds DeltaEncoder, a pure value-type encoder converting (timestamp, value) samples into a columnar delta format: absolute value at index 0, deltas thereafter, scaled to fixed-point integers at precision = 4. Gives 2.4–3.3× uncompressed size reduction.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference
• Add CHANGELOG entry for user facing changes
• Add Objective-C interface for public APIs - see our guidelines (internal)
• Run make api-surface when adding new APIs

[barboraplasovska]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Wire timeseries collection into RUM session and configuration #2885
• Add TimeseriesSessionCollector with memory and CPU collection #2884
• Add delta encoder for timeseries compression #2883 👈 (View in Graphite)
• Generate timeseries memory and CPU RUM event models #2882
• Fix ObjC bridge generator for nested DataPoint types #2881
• feature/timeseries

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