feat(17-01): ResolvedMember model, ISharePointGroupResolver interface, SharePointGroupResolver CSOM+Graph implementation

- ResolvedMember record in Core/Models with DisplayName and Login
- ISharePointGroupResolver interface with ResolveGroupsAsync contract
- SharePointGroupResolver: CSOM group user loading + Graph transitive AAD resolution
- Internal static helpers IsAadGroup, ExtractAadGroupId, StripClaims (all green unit tests)
- Graceful error handling: exceptions return empty list per group, never throw
- OrdinalIgnoreCase result dict; lazy Graph client creation on first AAD group

SharepointToolbox/Core/Models/ResolvedMember.cs

@@ -0,0 +1,10 @@
+namespace SharepointToolbox.Core.Models;
+
+/// <summary>
+/// Represents a resolved leaf member of a SharePoint group or nested AAD group.
+/// Used by <see cref="SharepointToolbox.Services.ISharePointGroupResolver"/> to return
+/// transitive member lists for HTML report group expansion (Phase 17).
+/// </summary>
+/// <param name="DisplayName">The display name of the member (e.g. "Alice Smith").</param>
+/// <param name="Login">The login / UPN of the member (e.g. "alice@contoso.com"), with claims prefix stripped.</param>
+public record ResolvedMember(string DisplayName, string Login);