Fix nightly library scaffold version selection #16490

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)


• This change requires a documentation update

Why

Nightly library initialization fails before project generation because its
bootstrap selects either the literal latest or an RN dist-tag without a matching
template. This also blocks the feed warmer's main-library closure.

Related: microsoft/react-native-windows#16489;
the failure predates that trigger-only change.

What

• 
  Checked bootstrap. Both callers share cRNL 0.63.0's supported RN 0.85.0
  bootstrap and check the exact RN/template pair through the configured registry,
  so dist-tags cannot select an incompatible scaffold.


• 
  Preserved targets. Stable versions stay unchanged. Nightly library and
  example dependencies still return to the requested RN/CLI versions before
  resolution; a failed branch does not discard healthy closures.


• 
  Regression coverage. Command-level tests and policy documentation cover
  shared selection, checkout paths, and explicit failures, so generator upgrades cannot
  silently reuse an unreviewed bootstrap.

Screenshots

Not applicable; no UI changes.

Testing

• 
  yarn test --scope @rnw-scripts/warm-feed - passed, 105 tests with inherited uppercase npm settings.


• 
  yarn workspace @rnw-scripts/warm-feed lint - passed; changed-file formatting was also checked explicitly.


• 
  yarn workspace @rnw-scripts/warm-feed build - passed.

Real Windows scaffolding with an empty npm cache and anonymous feed access
passed, including both nightly manifest rewrites, actual warmer collection, and
both dependency-graph resolutions. Full clean-agent/native CI validation is pending.

Changelog

Should this change be included in the release notes: yes

Use a compatible bootstrap when creating Windows libraries against nightly React Native.

Microsoft Reviewers: Open in CodeFlow