avoid Keys/Values ToList materialization in DictionaryWrapper

[SimonCropp]

The Keys/Values properties (both the generic ICollection and the
non-generic IDictionary surface) materialized a full List on every
access via Cast().ToList() / ToList(). Replace with a lazy
ReadOnlyCollectionWrapper that holds the source IEnumerable plus
a snapshot Count and defers enumeration to the caller.

Benchmark (net11.0 in-process, DictionaryWrapperKeysBenchmark):

Access only (no iteration):
  Hashtable.Keys      Count=500: 16,541 ns / 4160 B ->  43 ns /  80 B
  ReadOnly.Keys       Count=500:  4,060 ns / 4056 B ->  33 ns /  32 B
  IDictionary.Keys    Count=500:  3,735 ns / 4056 B ->  19 ns /  32 B

Access + foreach:
  Hashtable.Keys      Count=500: 17,530 ns / 4160 B -> 16,443 ns / 136 B
  Hashtable.Keys      Count=5  :    228 ns /  200 B ->    198 ns / 136 B

Allocations drop from O(N) to a flat 32-80 B regardless of dictionary
size. CPU is dominated by the underlying Cast iterator on the iterate
path, so wall-time wins there are modest; the pure-access path (e.g.
reflection / introspection callers that never enumerate) is 100-400x
faster at Count=500.