docs: trim core architecture guides and diagrams - #606

What

Trim the core architecture guides (docs 00-04) and the diagram caption files to terse descriptions of the current contract, per the #598 house standard: cut rationale essays, grant-milestone and platform-target planning material, before/after narrative, and duplicated primitive-by-primitive prose. 0.3 version promises are reduced to current fact. Net ~770 lines removed.

Why

These are the canonical what-is-true-now guides and had drifted: they carried planning prose the rubric bans, stale crate/interface vocabulary, and several claims the current code no longer backs. Owner boundaries restored: 05 owns the SDK, 08 owns the venue layer and platform model; the core guides link them rather than duplicating.

Testing

Documentation-only, pure markdown: no build required. Verified every retained claim against crates/nexum-runtime (engine_config.rs limit keys/defaults, wit/nexum-host world imports, host/local_store_redb.rs namespacing and transaction model), confirmed no em dashes, confirmed no cross-doc links point at removed anchors.

AI Assistance

Documentation-only sweep; claims verified against the runtime source, and the markdown link graph checked for dangling references.

Deviations flagged

• Local store is not per-event transactional. Docs 00/02/04 asserted an implicit per-event write transaction with all-or-nothing rollback on trap. crates/nexum-runtime/src/host/local_store_redb.rs (get/set/delete) opens and commits its own redb transaction per call, and host/impls/local_store.rs calls straight through with no supervisor-level wrapping, so a trap mid-event keeps already-committed writes. Docs corrected to per-call committed durability; the missing atomic-rollback guarantee is flagged here rather than papered over.
• Local store isolation model. Docs 04/00 described one redb file per module; the code is a single redb file with a 32-byte keccak256(module_name) key prefix per ADR-0003 (local_store_redb.rs). Corrected in the docs.
• shepherd:cow / cow-api superseded by the venue-adapter model. Docs 00/01/02 and both diagram files presented shepherd:cow/cow-api and a shepherd world as the shipped CoW contract. In the tree wit/shepherd-cow carries only cow-events, there is no cow-api host interface or OrderBookPool, and order submission is the videre:venue venue-adapter contract (wit/videre-venue, crates/cow-venue, crates/videre-sdk). Prose corrected to the venue model with a doc-08 pointer; the rendered diagram sources under docs/diagrams/*.mmd/*.png still show the old cow-api path and are marked historical pending regeneration (out of scope for a docs sweep).