[jiadebin]

What problem does this PR solve?

CSE LogOnly witnesses may temporarily lead Raft to repair a Full voter. PD and Scheduling must distinguish that state from a data-serving leader and from an actually empty Region. Explicit protocol support must be negotiated before admitting witnesses; the old is_witness bit is not sufficient.

When a Witness reaches critical Raft-disk pressure, accepting the next Append also requires a finite plan for the Full flush/checkpoint chain that will make the accepted data recoverable from DFS. A raw retained-byte hint cannot price future ChangeSets or bind retries to one immutable Full state.

What is changed and how does it work?

• Add immutable-role-independent Store protocol support and PD/Scheduling primary negotiation fields.
• Carry a presence-aware recovery status on PD and Scheduling heartbeats: temporary leader state, data-service readiness, reliable-size source, target peer and recovery session.
• Preserve the status through PD HA full/incremental sync using sparse identity-bound entries, avoiding nil/empty-message ambiguity for legacy Regions.
• Add session-bound witness log-pressure feedback used by the Store-wide LogOnly admission protocol.
• Add a versioned WitnessDrainPlan carried only after Full seals business input at applied == commit == last. It binds one UUID to the Full flush-ledger incarnation/revision, writable generation/write sequence, and overflow-checked aggregate future ChangeSet/L0 bounds plus the largest single ChangeSet bound needed to prove RFEngine's per-Ready encoding limit.
• Add explicit accepted-plan and Store-owner IDs in the response. Echoing a request is not admission, and a changed ledger/frontier requires a new plan identity.
• Keep existing field numbers and regenerate the Go/Rust/C++ bindings and protocol lock.

The drain plan is semantic evidence, not a physical IO ticket. The Witness companion must still combine it with its exact pending suffix and metadata projection and reserve every local WAL/rlog/MANIFEST copy before stepping the triggering Append. Production selector activation and the corresponding negotiated Store capability remain in the CSE draft.

The branch is synchronized with current kvproto master so the PD master draft keeps the upstream RU timeline schema while using the LogOnly protocol. Release-branch adoption should be handled as a separate backport.

Companion drafts:

• TiKV/CSE: https://github.com/tidbcloud/cloud-storage-engine/pull/6381
• PD: schedule: support fixed-role Full and LogOnly store pools tikv/pd#11370

Validation

• Protocol formatting and compatibility check passed.
• go test ./pkg/raft_serverpb passed, including nested plan presence and round-trip identity.
• Current-head GitHub Go, Rust and C++ jobs passed.
• The CSE companion compiled against this exact revision; its debt/ShardMeta-bound tests passed 3/3, plan construction/validation and future flush quote tests passed 3/3, and full-workspace make clippy passed with zero warnings.

Check List

• Generated Go bindings and protocol lock
• Protocol formatting/compatibility checks
• Go/Rust/C++ CI
• Cross-component focused tests in the companion draft

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