docs: add OCR2 protocol walkthrough

docs/ocr2-protocol-walkthrough.md

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+# OCR2 Protocol Walkthrough
+
+This document is a high-level guide to a single OCR2 report generation and
+transmission flow. It is meant as a visual starting point for readers who want
+to connect the protocol messages to the implementation files.
+
+For protocol details and edge cases, refer to the OCR whitepapers listed in the
+repository README.
+
+## Flow
+
+```mermaid
+sequenceDiagram
+    participant L as Leader oracle
+    participant O as Follower oracles
+    participant F as Report finalization
+    participant T as Contract transmitter
+
+    L->>O: MessageObserveReq(query)
+    O-->>L: MessageObserve(signed observation)
+
+    Note over L: Wait until > 2f observations
+
+    L->>O: MessageReportReq(attributed observations)
+    O-->>L: MessageReport(signed report)
+
+    Note over L: Wait until report quorum
+
+    L->>O: MessageFinal(attested report)
+    O->>F: EventFinal
+    F->>O: MessageFinalEcho(attested report)
+    F->>T: EventTransmit
+    T->>T: ContractTransmitter.Transmit(report, signatures)
+```
+
+At a high level:
+
+1. The leader requests observations from all oracles.
+2. Each oracle observes application-specific data and signs its observation
+   offchain.
+3. The leader verifies the signed observations and waits for more than `2f`
+   observations.
+4. The leader broadcasts the attributed observations and asks oracles to
+   construct a report.
+5. Oracles construct and sign the report.
+6. The leader waits for report quorum and broadcasts a finalized attested
+   report.
+7. Report finalization emits a transmission event.
+8. The transmission protocol calls `ContractTransmitter.Transmit`.
+
+## Leader Phases
+
+```mermaid
+stateDiagram-v2
+    [*] --> Observe
+    Observe --> Grace: > 2f observations
+    Grace --> Report: DeltaGrace elapsed
+    Report --> Final: report quorum reached
+    Final --> Transmission: MessageFinal accepted
+    Transmission --> [*]: Transmit called
+```
+
+These phases correspond to the `phaseObserve`, `phaseGrace`, `phaseReport`,
+and `phaseFinal` states in the OCR2 report generation leader implementation.
+
+## Code Map
+
+| Step | File | Function |
+| --- | --- | --- |
+| Start a leader round | `offchainreporting2plus/internal/ocr2/protocol/report_generation_leader.go` | `startRound` |
+| Handle an observation request | `offchainreporting2plus/internal/ocr2/protocol/report_generation_follower.go` | `messageObserveReq` |
+| Sign an observation | `offchainreporting2plus/internal/ocr2/protocol/observation.go` | `MakeSignedObservation` |
+| Verify observations at the leader | `offchainreporting2plus/internal/ocr2/protocol/report_generation_leader.go` | `messageObserve` |
+| Enter the report phase | `offchainreporting2plus/internal/ocr2/protocol/report_generation_leader.go` | `eventTGraceTimeout` |
+| Build and sign a report | `offchainreporting2plus/internal/ocr2/protocol/report_generation_follower.go` | `messageReportReq` |
+| Collect report signatures | `offchainreporting2plus/internal/ocr2/protocol/report_generation_leader.go` | `messageReport` |
+| Finalize an attested report | `offchainreporting2plus/internal/ocr2/protocol/report_finalization.go` | `finalize` |
+| Decide whether to transmit | `offchainreporting2plus/internal/ocr2/protocol/transmission.go` | `eventTransmit` |
+| Transmit the report | `offchainreporting2plus/internal/ocr2/protocol/transmission.go` | `eventTTransmitTimeout` |
+
+## Reporting Plugin Callbacks
+
+OCR2 handles networking, signatures, quorum, finalization, and transmission.
+Application-specific behavior lives behind the `ReportingPlugin` interface in
+`offchainreporting2plus/types/types.go`.
+
+During a successful round, these callbacks are typically involved:
+
+| Callback | Purpose |
+| --- | --- |
+| `Query` | Creates the query sent by the leader in `MessageObserveReq`. |
+| `Observation` | Reads application-specific data and returns an observation. |
+| `Report` | Builds the report from attributed observations. |
+| `ShouldAcceptFinalizedReport` | Decides whether a finalized report should enter the local transmission schedule. |
+| `ShouldTransmitAcceptedReport` | Runs immediately before `ContractTransmitter.Transmit`. |
+
+## Minimal Message Path
+
+The happy path for one OCR2 round can be read as:
+
+```text
+MessageObserveReq
+MessageObserve
+MessageReportReq
+MessageReport
+MessageFinal
+EventFinal
+MessageFinalEcho
+EventTransmit
+ContractTransmitter.Transmit
+```
+
+The real implementation includes additional handling for timeouts, leader
+changes, persistence, duplicate messages, malformed messages, and Byzantine
+faults. Those paths are intentionally omitted here so this walkthrough can stay
+focused on the core report path.