Optimize ReaderWriterLockSlim  allocations

PR #12264 added a per-instance  `ReaderWriterLockSlim`  to  `PropertyDictionary`  to prevent concurrent modification during enumeration. Because  `PropertyDictionary`  is created frequently during evaluation and  `ProjectInstance`  construction, this adds a relatively large lock object to every instance.

In an OrchardCore design-time build,  `ReaderWriterLockSlim` account for approximately 71.84 MB of allocations across six MSBuild worker processes. Almost all are attributed to this per-instance allocation.

Start by reviewing PR #12264 and tracing how PropertyDictionary is created during evaluation and ProjectInstance construction. Measure the per-instance ReaderWriterLockSlim allocations in a representative design-time build; done means reducing that allocation cost while preserving safe concurrent enumeration.

Written by the indexing model from the issue text.