test(runtime): define fault injection contract

[luoye520ww]

Problem

The reliability plan needs deterministic fault injection for crash, SSE, network, storage, and credential-store scenarios, but there is no constrained contract to keep test faults out of normal behavior.

Scope

Adds a fixed fault-kind allowlist, disabled-by-default spec normalization, one-shot/repeated activation semantics, and a bounded delay (0-60 seconds). Trigger checks revalidate untrusted specs and fail closed.

Non-goals

No production runtime hooks, environment activation, network interception, storage mutation, or UI are included. Those are separate integration PRs.

Tests

• npm.cmd --prefix kun test -- tests/fault-injection.test.ts (9 passed)
• npm.cmd run typecheck
• npm.cmd --prefix kun run typecheck
• npm.cmd run lint
• npm.cmd run build
• git diff --check

Review

Completed functional, lifecycle, data integrity, security, cross-platform, compatibility, and scope review. No package smoke is claimed because this PR contains a pure test contract.

Issue

Part of #885

[luoye520ww]

This is superseded by #953, which keeps the fault-injection contract and wires it into the bounded Node HTTP/SSE runtime boundary with full typecheck, test, Linux, Windows, and macOS validation. Closing the contract-only PR to avoid duplicate review.