refactor: complete removal of go-linq/x/exp, modernize Go syntax

[AWaterColorPen]

Summary

Completes the removal of legacy dependencies started in the previous PR, and modernizes Go syntax throughout the codebase.

Changes

Dependency removal:

• Remove all remaining go-linq usage in server.go, server_policy.go, server_user_domain.go, server_user_role.go
• Replace golang.org/x/exp/constraints.Ordered with stdlib cmp.Ordered in Diff()
• go-linq and golang.org/x/exp fully removed from go.mod after go mod tidy

Stdlib replacements:

• linq.Distinct → stdlib-based distinct() generic helper (already in inheritance.go)
• sort.Slice → slices.SortFunc in EdgeSorter.RootFirstSort and LeafFirstSort
• DiffPolicy key map changed from map[any]*Policy to map[string]*Policy (type-safe)

Bug fix:

• getSourceFeatureG2: initialize BFS queue with DefaultFeatureRootName (was empty before)

Readability:

• Added // BFS: comments on growing-queue for-loops explaining the pattern

Spec:

• Updated modernization spec to mark completed tasks

Test results

ok  github.com/awatercolorpen/caskin       8.292s
ok  github.com/awatercolorpen/caskin/playground  0.615s

Part of Phase 1 modernization plan.

[AWaterColorPen]

AI Review — LGTM ✅

Clean modernization of the dependency removal. A few observations:

Strengths:

• Diff[T] now uses explicit map lookups instead of go-linq reflection — clearer semantics and better performance.
• LeafFirstSort reversal via cmp.Compare(e[b.V], e[a.V]) is correct (note: reversed b, a to achieve descending order).
• distinct[T] defined in inheritance.go is a well-written generic helper, consistently reused in server_policy.go, server_user_domain.go, and server_user_role.go.
• BFS comments added in object_directory.go, server_check.go, and server_feature_reset.go — good readability improvement.
• go-linq and golang.org/x/exp fully removed from go.mod/go.sum. ✅
• CI passes. ✅

Minor note:
The server_policy.go diff has a stray blank line after package caskin (empty import block left behind). Not a blocker but worth a quick cleanup.

Ready to merge once you are happy with it.