intent: WIT packages, venue-adapter world, pool router

[mfw78]

Problem. Doc 09 step 3a (PR #132): the intent core does not exist. No nexum:value-flow or nexum:intent packages, no second component kind, no pool router, no venue SDK.

Scope. The nexum:value-flow and nexum:intent WIT packages at 0.x (identifier-hygiene rule: check every id against WIT keywords and the nine binding targets). The venue-adapter world with a second bindgen and instantiation path, manifest kind venue-adapter, [[adapters]] engine config, and per-adapter scoped transport grants. The pool-router host implementation: venue-id resolution, derive-header then guard-seam then submit sequencing, per-adapter call serialisation (Store is not Sync), per-caller quotas charging decode failures to the caller. The intent-status event variant with router-driven status polling. nexum-venue-sdk (VenueAdapter trait, #[nexum::venue] macro, borsh IntentBody derive, typed transport wrappers, and the typed intent client core) and nexum-venue-test (codec round-trip vectors, header-derivation goldens, MockTransport). The echo-venue example pair as tutorial and conformance target.

Layering. This epic ships the two portable WIT layers every venue module imports: the venue-agnostic intent vocabulary (nexum:intent, nexum:value-flow) and the nexum:adapter venue-adapter world, over the runtime host ABI (nexum:host). No keeper piece migrates here; #135's keeper mirrors these types SDK-side per ADR-0009.

Dependencies. De-cow epic (M0). #51 (capability allowlist surface), #107 (fuel-accounting verification underwrites the metering-attribution claims).

Notes. The adapter world carries the requires-signing(unsigned-tx) submit variant from day one (owner decision for N=2 on-chain settlement): submit may return a transaction the host routes through the guard's host-signed class instead of a receipt. Adapters still structurally cannot move value.

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Next-train plan

Design gate (train step 0). Merge PR #132 (renumber the egress-guard ADR 0011 to 0012, closes #131).

Current state. Essentially nothing of the intent core exists: no nexum:value-flow/nexum:intent WIT, one component kind wired (EventModule), no pool router, no [[adapters]], no proc-macro crate, no borsh. Foundations that exist: the de-cow refactor landed (engine core domain-free, cow in shepherd-cow-host) and the extension seam (crates/nexum-runtime/src/host/extension.rs) is compatible with the per-extension router the pool needs. Children #226 through #234 are in review on the M1 train.

Children (ordered):

• wit: author the nexum:value-flow types package (0.1) with an identifier-hygiene gate #226 — nexum:value-flow WIT 0.1 + identifier-hygiene gate · Wave A · M
• wit: author the nexum:intent package (types + pool interface) with the requires-signing submit outcome #227 — nexum:intent WIT (pool iface, requires-signing outcome) · Wave B · M · deps wit: author the nexum:value-flow types package (0.1) with an identifier-hygiene gate #226
• runtime: venue-adapter world, manifest module-kind, and [[adapters]] engine config with scoped transport grants #228 — venue-adapter world + manifest kind + [[adapters]] config · Wave C · L · deps wit: author the nexum:intent package (types + pool interface) with the requires-signing submit outcome #227
• host: implement the nexum:intent pool-router (derive-header -> guard-seam -> submit, serialization, quotas) #229 — pool-router host iface (derive-header, guard-seam, submit, quotas) · Wave D · L · deps runtime: venue-adapter world, manifest module-kind, and [[adapters]] engine config with scoped transport grants #228
• sdk: ship nexum-venue-sdk (VenueAdapter trait, borsh IntentBody derive, typed transport wrappers) #230 — nexum-venue-sdk (VenueAdapter trait, borsh derive, typed transports, intent client core; classification here is orderbook-submit only, never off-chain poll decode) · Wave D · M · deps runtime: venue-adapter world, manifest module-kind, and [[adapters]] engine config with scoped transport grants #228
• runtime: add the intent-status event variant and router-driven status polling #231 — intent-status event variant + router-driven status polling · Wave E · M · deps host: implement the nexum:intent pool-router (derive-header -> guard-seam -> submit, serialization, quotas) #229
• sdk: add the #[nexum::venue] proc-macro (adapter cdylib glue + per-component world) #232 — #[nexum::venue] proc-macro (adapter glue + per-component world) · Wave E · M · deps sdk: ship nexum-venue-sdk (VenueAdapter trait, borsh IntentBody derive, typed transport wrappers) #230, sdk: scaffold nexum-macros and ship #[nexum::module] over the current world #223
• sdk-test: nexum-venue-test conformance kit (codec vectors, header goldens, MockTransport) #233 — nexum-venue-test conformance kit (codec vectors, header goldens, MockTransport) · Wave E · M · deps sdk: ship nexum-venue-sdk (VenueAdapter trait, borsh IntentBody derive, typed transport wrappers) #230
• examples: echo-venue adapter + module pair (tutorial and conformance target) #234 — echo-venue example pair, end to end · Wave F · M · deps runtime: add the intent-status event variant and router-driven status polling #231, sdk: add the #[nexum::venue] proc-macro (adapter cdylib glue + per-component world) #232, sdk-test: nexum-venue-test conformance kit (codec vectors, header goldens, MockTransport) #233

#234 (echo-venue) is the first-train cut line: it proves the intent core end to end before the CoW adapter (#138) lands.

Blocked by: #136 (macro). Blocks: #138.