ci: reduce Windows test partitions

[wan9chi]

What changed

• Reduce default Windows nextest partitions from six to two.
• Keep the Node-backed ignored tests in their dedicated third Windows job.

Motivation

With Namespace Windows runners, the six default partitions complete only 88-111 tests each in 1.7-5.1 seconds, while job setup dominates and the seventh Windows job spills into a second runner wave. A workflow-dispatch comparison with two default partitions completed 299 and 283 tests in 6.3 and 4.9 seconds, reduced aggregate default Windows job time from about 113 seconds to 34 seconds, and remained well ahead of the musl critical path.

[wan9chi]

• ci: reduce Windows test partitions #530 👈 (View in Graphite)
• main

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[wan9chi]

Merge activity

• Jul 9, 1:56 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Jul 9, 1:56 PM UTC: @wan9chi merged this pull request with Graphite.