Swap the mock engine to `@parity/truapi-host`

[decrypto21]

Summary

Replace the mock's wire engine - hand-written @novasamatech/host-container - with the generated @parity/truapi-host dispatcher. Keep all value-add (dev-key derivation, real sr25519 signing, signing log, productAccounts pinning, RPC proxy, Playwright fixture, fault injection). Only the protocol layer changes.

Why now

product-sdk#186 goes direct to @parity/truapi, deleting @novasamatech/host-api(-wrapper). On merge, a migrated product emits the generated truapi envelope the current mock can't decode - so every migrating product loses its E2E harness unless the mock moves in lockstep.

Design

Swap flat container.handleXxx(...) registration to nested, versioned, tagged-union handlers via createTrUApiServer(provider, handlers). ~40 slots regroup under 15 services; legacy error classes map to generated tagged unions. The host-side iframe Provider (already built, src/truapi-engine/host-iframe-provider.ts) plugs straight in. The fault layer wraps the raw provider below the engine, so it's retained - and protocolVersion skew becomes implementable here.

Tasks

• Drop in the host-side iframe Provider (built)
• Replace createContainer with createTrUApiServer(provider, handlers); regroup ~40 slots into 15 nested services; map legacy errors → tagged unions
• Port the chain.* set (14 handlers) - reuse the truapi-host-integration/chain-handlers.ts prototype (response/follow-item mapping to finish)
• Rewrite the test product on the @parity/truapi client (the chunkiest part - it currently uses @novasamatech/host-api-wrapper)
• Re-target the fault layer to truapi's wire (handshake = wire id 1 via decodeWireMessage); implement protocolVersion
• Drop @novasamatech/host-container; bump major; update CHANGELOG/forum-post/README

Acceptance criteria

• A product on migrated @parity/product-sdk-* E2E-tests green against the new-engine mock; existing integration tests pass; @novasamatech/host-container removed.

Sequencing

Must land in lockstep with / after product-sdk#186