roadmap: Cap'n Proto IPC schema finalisation (Stage 5a — OWNER-GATED, needs Opus sign-off) #194

Origin

docs/ROADMAP.md Stage 5 ("Distributed & fast — scale + specialised hardware"), row 5a:

Stage 5  Distributed & fast                 scale + specialised hardware
    5a  Cap'n Proto IPC                     Rust ↔ Julia, :8090

And the endpoint-target row:

| "Cap'n Proto serialisation" | 0 .capnp files | crates/echidna-wire/ contains schemas for ProofState / Goal / Tactic / EmbeddingRequest / RankingResponse; IPC on :8090 is Cap'n Proto; JSON retained only as debug fallback |

Handover document: docs/handover/L1-CAPNPROTO-PROMPT.md.

Scope

OWNER-GATED — design needs Opus sign-off before any implementation:

• crates/echidna-wire/ with .capnp schemas for the 5 named messages (ProofState, Goal, Tactic, EmbeddingRequest, RankingResponse)
• Replace the current JSON-over-HTTP IPC to Julia on :8090 with Cap'n Proto framing
• Keep JSON as a debug fallback only
• Schema versioning convention so Rust and Julia can evolve independently

Estimated PR count: small for schema, larger for the JSON→Cap'n Proto cutover and dual-mode period.

Dependencies

• OWNER (Opus): finalise the schema before any code lands. The handover prompt at docs/handover/L1-CAPNPROTO-PROMPT.md is the entry point
• L2.3 Chapel cancel-token preemption (sister tracker — see filed companion issue) is HARD-GATED on this landing
• L2.4 / L2.5 / L2.6 / L2.7 Chapel sub-waves are also hard-gated on this
• The Verisim schema agreement (separate tracker) is independent but lives in the same Stage-5 prerequisite cluster

Why filed

To make Stage 5a tracker-visible without implying any implementation will begin before owner sign-off. The handover doc exists but is not currently visible as an open issue; this lets the owner (or a future Opus session) drop the green-light directly on the tracker.