feat(mock): re-port MockPlatform + createMockHost onto current core

[decrypto21]

What

Re-port of the TrUAPI mock host onto current main. Supersedes the stale #258 branch (feat/mock-platform), which was on a superseded base (worktree-issue-96-rust-core-port) and written against #104's API. Main already landed the core, so this brings over only the mock-specific additions, adapted to main's core API — one clean commit on main.

Refs #257 (in-core mock host: MockPlatform + createMockHost). Part of the testing ladder tracked in sdk-team#43.

What's in it

• truapi-platform — MockPlatform behind the mock feature: in-memory storage, fixed-policy permissions, configurable chain, notifications, confirmations, theme, and lookup-only preimages. PreimageHost is lookup-only (the core owns Bulletin submission), so submit_preimage / BulletinAllowanceSigner are dropped and content is seeded via insert_preimage. Types come from truapi::latest per crate convention.
• truapi-host — createMockHost returns the nested RequiredHostCallbacks shape (main's per-capability grouping, not #104's FlatHostCallbacks); mockRuntimeConfig sets the new required bulletin genesis hash.
• wasm-bridge test — drives the real WASM core through the mock callbacks, no browser.
• CI — @parity/truapi-host job builds the WASM and runs bun test with REQUIRE_WASM=1, wired into the ci-status gate.

Deliberately not ported

The through-core mock-wallet harness from #258 is left out: main already provides the equivalent StubPlatform + SSO statement/frame builders in truapi-server/src/test_support.rs. Converging those two doubles into one canonical mock is tracked as a follow-up (sdk-team#43-D8 / D7).

Tests

Rust truapi-platform mock tests + truapi-host create-mock-host and wasm-bridge (real WASM core) suites green. Dev-only: the mock feature is excluded from production builds.

Closure

Closes the re-port half of #257. The browser E2E harness (#260 / PR #261) rebases on top once this lands. See sdk-team#43 for the full stack and merge order.