Split the monolithic runtime test suites into domain-owned shards

Context

The line-count ratchet introduced by #997 identifies the three largest test files as:

• packages/runtime/test/runtime.test.ts: 21,376 measured lines (target: 1,000)
• packages/runtime/test/artifact-gates.test.ts: 13,881 measured lines (target: 1,000)
• packages/runtime/test/generated-workflow-verifier.test.ts: 9,212 measured lines (target: 1,000)

The files provide valuable integration coverage, but their size makes ownership, selective execution, review, and failure diagnosis expensive. The runtime file also has module fan-out 22 (target: 15).

Proposed work

• Inventory test domains and shared fixture boundaries before moving cases.
• Split by behavior/contract, retaining explicit integration shards for cross-domain scenarios.
• Extract typed fixture builders without hiding important assertions in generic helpers.
• Preserve Node/Bun selectors, timeouts, and release-validation lane coverage.

Acceptance criteria

• No new shard exceeds 3,000 measured lines as an intermediate ceiling.
• The original three files each shrink by at least 40% in the first milestone.
• Node, Bun adapter, coverage, and release-validation test inventories remain equivalent.
• Metric and fan-out baselines are lowered in the same PR.

Follow-up to #997.

Start by inventorying domains and fixture boundaries across packages/runtime/test/runtime.test.ts, artifact-gates.test.ts, and generated-workflow-verifier.test.ts. Read the existing Node, Bun adapter, coverage, and release-validation selectors before planning shards. Done means no shard exceeds 3,000 measured lines, the three originals shrink by at least 40%, test inventories remain equivalent, and metric and fan-out baselines decrease.