[DS-3860] whole round snapshot consistency

[brunotm]

on v31 is possible to fully defer definitions updates to the next round as state doesn't have to be carried by the previous outcome.

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[cl-efornaciari]

Deferred channel definitions: changes agreed in round N are persisted to c/defs immediately but only take effect at N+1, so Observation, StateTransition and Reports always operate on the exact definitions (and opts) the round's stream values were gathered under. This closes the v30-inherited inconsistency where a mid-round definition change could produce reports encoded under definitions/opts that the observations behind them did not match. The effective/pending two-set construction is clean, carrying channelStateSeqNr in the precursor keeps Reports self-sufficient (and wire-compat with the LLOOutcomeProtoV1 layout is preserved), and the test coverage of the new semantics — addition effective at N+1 / first reportable at N+2, deferred removal, opts tracking, SyncTo watermark gating — is thorough. Build + go vet + v31/protocol suites pass locally.

One concern I consider blocking, detailed inline: the shared OptsCache is now written by two paths at different seqNrs (channelCache.put from the store, Reports from the precursor) while being read mid-round by StateTransition (ProcessCalculatedStreams, isReportable) and Reports (reportableChannels, codecs). After a defs-change round, a late or overlapping Reports call can leave the cache pinned at an older seqNr indefinitely (the cache-hit path never re-syncs) or flip it mid-round — making StateTransition/Reports outputs depend on node-local interleaving, which conflicts with the ReportingPlugin determinism contract (memoization side effects must be unobservable). Concrete interleavings and suggested fixes inline.

Also one doc nit on applyChannelVotes.