MCP Proposal Creation

[larskuhtz]

Stacked on #3225 (lars/proposer-election). Adds the proposal-creation component and wires proposals end to end through the sim: slots finalize with Positive entries carrying the scheduled proposers' payload roots, across rotations, vacancies, and a window rollover.

Design

Proposal creation is its own component, outside the consensus core. Proposing is driven by the slot deadline, not by eligibility: a proposal only counts if it is disseminated and decoded by a supermajority before the slot deadline D_s, and a proposer benefits from sealing as late as viable. The planner seals at deadline − lead, with lead a fixed conservative bound for now; the component boundary — facts in, wake and seal commands out — is the seam where adaptive or payload-economic strategies plug in later without touching consensus.

Everything else is a constraint from the MCP spec (main.tex, v1.2.0): a party that does not yet have B_{r−y} and state^{r−x} does not propose in round r, with x the execution delay and y ≤ x the observation cutoff.

• Chaining gate (B_{r−y}): slot r is sealed only once slot r − y is part of the contiguous finalized prefix reported by the conductor — the stand-in for the translated block until sequencing exists. A lagging chain holds seals back; a seal skipped past its deadline leaves the index empty, a throughput cost and never a consensus hazard.
• Blind-window knowledge: the in-flight proposals a proposer must know at its own index it has structurally — mid-phase they are its own, and at a handoff the schedule's rotation vacancy of y slots guarantees there are none. The planner sees vacancy simply as "not scheduled".
• Execution prerequisite (state^{r−x}): not modeled; it slots in beside the chaining gate once execution progress exists as an input.

Changes

• monad-mcp-chorus/src/proposing.rs: ProposalPlanner — requests a wake at deadline − lead for each slot where the schedule assigns this node an index; on wake it seals, or gates on chaining and releases or skips at the next chain advance.
• monad-mcp-chorus/src/da.rs: the proposer-side seam towards the DA layer — DataAvailability::submit_proposal, by which the planner hands a sealed payload over for dissemination, plus NullDa for proposal-less tests. The consensus-side direction is the existing ChorusDAEvent / ChorusDACommand seam.
• Fact surface: MonadConductor emits the (previously never-emitted) CloseSlots output whenever the completion cap advances; the runtime forwards OpenSlots/CloseSlots to the observer as handle_slots_opened/handle_chain_advance, and observers compose as tuples.
• Runtime gains the DA-event entry point — an associated DAEvent and handle_da_event — so a node's DA layer can report availability to the slot instance it drives. Runtimes without a DA layer default it away.
• monad-mcp-chorus-sim: MockDa, an in-memory DA mock. A submission becomes available locally at once and is announced as one message over the simulated network, under the same latency model as consensus traffic; availability reaches consensus as ChorusDAEvents. The mock validates announcements with the HeaderAuth consensus itself votes with, so the two cannot disagree about who proposes where. SimMessage wraps the wire format, and add_proposer_node wires planner, mock DA, and a per-node observer.
• Entry, FastCommitVote and FastCommitQc become pub, so finalization data is inspectable by consumers — as ledger sequencing eventually will.

Testing

• Planner unit tests: wake timing, lead underflow, gating, release and skip, genesis exemption, non-proposer slots. MockDa unit tests: submit, announce, availability reporting, idempotent delivery, first root wins, and an announcement for an index its signer does not hold.
• tests/proposals.rs end to end: K = 1 rotating every slot — every entry finalizes Positive with the scheduled proposer's payload root, on the same finalization schedule as proposal-less runs; K = 2 staggered phases with y = 2 across a window rollover — genesis exemption, rotation vacancies (Negative exactly where the schedule says), and the chaining gate under load.