[woksin]

Summary

The second set of Code Health improvements for the Scene engine. Behavior does not change: each file was pinned by characterization specs before it was refactored, in both the .NET and TypeScript engines.

Changed

• Blueprint package verification is split into focused rule sets for metadata, placement, screen sets and templates in both engines, with shared fixtures pinning its diagnostics and their order (part of Code Health: raise all files to 9.5 #294)
• Package list editing is split into focused helpers in both engines, with shared fixtures pinning its results and guarding against mutating the input (part of Code Health: raise all files to 9.5 #294)
• Package version range evaluation is split into focused steps in both engines, with shared fixtures pinning its results (part of Code Health: raise all files to 9.5 #294)
• Profile blocker rules and legacy profile migration are split into focused steps in both engines, with shared fixtures pinning sparse profile fixes (part of Code Health: raise all files to 9.5 #294)
• Shell resolution and shell switch planning are split into named steps in both engines, with shared fixtures pinning their edge cases (part of Code Health: raise all files to 9.5 #294)

[woksin]

Reviewer notes for this PR (batches 5 to 7a of the Scene Code Health work in #294).

• Behavior-preserving refactors only. Each refactor follows characterization specs, and the .NET and TypeScript engines share the same fixture corpus. Most of the added lines are fixture JSON.
• Local gate on head 4f53aad: dotnet format --verify-no-changes, Release and Debug builds, Engine and Model specs on net11 and net10, yarn ci, yarn verify-packages, yarn verify-blueprints, Storybook build, node --test and actionlint all passed. CI runs the browser specs and the blueprint scaffold.
• Each batch got its own review, and the whole change got a review with a security lens. None reported a blocking finding.
• Code Health went up in BlueprintVerification, verifyBlueprintPackage, PackageList and packageList, ProfileBlockers and findProfileBlockers, packageVersionRange, ShellResolver and resolveEffectiveShell, and ShellSwitchPlanner and planShellSwitch. ProfileMigrator.cs (9.09), PackageVersionRange.cs (9.38) and migrateLegacyProfile.ts (9.38) stay below 10. They are left as they are because Code Health sweeps are paused.
• Commit 1fa0e8b on its own fails a shell resolver spec. The next commit, 8dbd17a, fixes it, so the branch head is green. Keep this in mind if you bisect.
• The rest of Code Health: raise all files to 9.5 #294 is paused by a change in priorities.