Adopt the SDK-style CoreLibrary project as primary

[danielmeza]

Description

CoreLibrary is the root of the dependency graph and a special case for the migration. An SDK-style CoreLibrary.Sdk.csproj (targeting netnano1.0) exists on the move-to-sdk branch, and the published v2 previews already restore natively against netnano1.0, but the SDK-style project is not yet the primary project for the repository. Because every other library depends on CoreLibrary, it has to be migrated and republished cleanly before (or at the front of) the fleet, so dependents can restore against an SDK-style, netnano1.0-targeted CoreLibrary.

How to solve the problem

Adopt the SDK-style CoreLibrary as the primary project and validate it as the leaf of the dependency graph:

• Make CoreLibrary.Sdk.csproj (or the SDK-style equivalent) the primary project, building/packing against the published nanoFramework.NET.Sdk.
• Confirm it produces the corlib package correctly (mscorlib bootstrap; see the Corlib SDK variant in the SDK design) and that a downstream library restores and builds against it.
• Republish the package targeting netnano1.0 so dependents no longer fall back to net to restore.

This is maintainer-owned (it lives in the CoreLibrary repo) and is the natural leaf-first first step of the fleet.

Acceptance: the SDK-style CoreLibrary is the primary project, builds/packs against the published SDK, publishes targeting netnano1.0, and a downstream library restores and builds against it.

Describe alternatives you've considered

• Keep the legacy CoreLibrary project as primary and migrate only leaf libraries. Rejected: everything depends on CoreLibrary, so the fleet cannot fully restore against SDK-style packages until CoreLibrary is republished against netnano1.0.

Additional context

• Specification: the Corlib SDK variant in 02 — SDK design §2.6 (component C10) and the leaf-first ordering in 09 — implementation strategy.
• Coverage and status: EXECUTION-PLAN.md — spec coverage matrix.
• Parent epic: SDK-style MSBuild project system for .NET nanoFramework #1784.
• Scope: CoreLibrary repo; maintainer-owned. Leaf of the fleet; precedes the broader lib-* migration.