fix(ci): bring Windows test matrix under five minutes

[pacphi]

Summary

• Reuse closed AQE 3.14.4 SQLite schema templates for private merge-test stores; batch fixture inserts in a transaction and verify schema, FTS, integrity, and the fresh-store import path.
• Split the 36 existing AQE merge tests into three independent files so Node can run them concurrently. Preserve their assertions, including the Windows EBUSY case.
• Run the full suite on every OS/Node matrix leg. Keep local coverage enabled by default and enforce the 70/70/70 gate on Ubuntu/Node 24 in CI. Update ADR-0014 and the issue plan.

Related to #262. This PR does not automatically close the issue.

Evidence at c54f6189

Both the workflow-dispatch run and the PR-triggered run passed all 13 jobs. Windows test-job elapsed times, measured from job start to completion:

Node	Before	Dispatch run	PR run
22	12:35	3:35	4:22
24	11:28	3:30	3:30
26	12:56	3:27	3:21

The Ubuntu/Node 24 coverage gate reported 94.31% lines, 83.17% branches, and 93.44% functions. Locally, the 36 merge tests fell from 6.4 seconds to 0.63 seconds on macOS/Node 26.

Local checks passed: full pnpm test with coverage and with AK_TEST_COVERAGE=0 under the strict real-state tripwire; focused merge and runner tests; UI tests; typecheck; lint and Markdown lint; build check; dispatch-surface tests; and actionlint.

Remaining verification

These are two completed runs at the exact PR head. Issue #262 calls for ten consecutive runs to establish the median and variation. Keep that stability check visible before treating the issue as closed or the branch as merge-ready.