docs: trim platform generalisation and linker seam to current contract

[mfw78]

What

Trim the two platform-design docs to a terse description of the shipped contract.

docs/08-platform-generalisation.md (1015 -> 312 lines): keep the Layer 1/2/3 world model, the universal interface signatures (verified against wit/nexum-host), the venue-adapter Layer 3 mechanics, and the complete WIT package layout. Collapse the mobile, WebView, and super-app targets and their per-primitive implementation tables to one sentence each. Cut the motivation essay, the "key insight", the host adapter specification, and the summary. Reduce the 0.2 status banner to one line, and point the SDK and six-primitive taxonomy at their owning docs (05 and 00).

docs/design/linker-extension-seam.md (121 -> 89 lines): rewrite against the current Extension trait. Drop the retired cow-api / shepherd-cow-host / CowBackend example, and describe the trait's real members (namespace, capabilities, link, service, provider, admit predicates, event sources), the HostServices and ExtState reach paths, and the videre venue platform as the live extension the shepherd binary registers.

Why

Both docs were design essays written before the code landed and had drifted from dev/m1: they described the pre-venue-adapter cow-api world extension and an Extension struct that is now a trait, and they reproduced stale WIT. Part of the #598 cruft sweep.

Testing

Documentation only, no build. Verified every retained WIT block and behavioural claim against wit/ and crates/ at the dev/m1 tip; confirmed no other doc links to the trimmed sections (the #layer-3-domain-extensions-venue-adapters anchor is preserved for docs 00 and 05). Purged em dashes.

AI Assistance

Documentation-only sweep; claims verified against the worktree at the dev/m1 tip, no build required.

Deviations flagged

Substantive mismatches between the prior doc 08 and the current reference-server host, now stated at actual behaviour in the doc:

• chain does not delegate signing to identity. The old doc claimed chain::request handles eth_sendTransaction by calling identity::sign and broadcasting the signed transaction. The reference host does the opposite: crates/nexum-runtime/src/host/impls/chain.rs:19-25 resolves only a read-only method surface and refuses every signing or mutating method (eth_sendTransaction, eth_sign, personal_sign, eth_sendRawTransaction, eth_accounts) with a denied fault (chain.rs:319-336).
• identity is an unimplemented stub. The old doc described a working key-management interface backed by Keystore/KMS/HSM. crates/nexum-runtime/src/host/impls/identity.rs:10-28: accounts() returns an empty roster and sign / sign-typed-data return the unsupported fault; a keystore backend is deferred.