[chain-pr 10/12] Rewrite Session Replay recording engine on the new change-encoder pipeline

[johnHackworth]

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

Changes

Move startRecording → datadogRecorder, profiler chunk → datadogProfiler. Drop V1 serialization (serializeNode/serializeMutations/serializeFullSnapshot/serializeStyleSheets and conversions/v* helpers + spec helpers). Adopt change-encoder-only flow: createChangeEncoder/Decoder enforce string-table soft cap, serializeFullSnapshotAsChange/serializeMutationsAsChange/serializeNodeAsChange replace V1 paths, takeFullSnapshot/takeNodeSnapshot rebuilt on top, recorderApi/postStartStrategy/preStartStrategy use computeSessionReplayState and new SessionManager. Drop isFullSnapshotChangeRecordsEnabled/isIncrementalSnapshotChangeRecordsEnabled feature flags. Update internal API, datadogRecorder.spec, recorderApi.spec, getSessionReplayLink, segmentCollection, startSegmentTelemetry to assume Change records. Update test helpers (serialization.specHelper, serializeHtml.specHelper, record/changes, record/segments) and remove obsolete fixtures (mutationPayloadValidator, vDom.specHelper, htmlAst.specHelper, vNode/vDocument/vStyleSheet/changeConverter/serializationVerifier).

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

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