Angular 21

[dhilt]

Upgrade ngx-ui-scroll ecosystem to Angular 21 (library, demo app, tests, and related tooling/dependencies).

As part of the migration, fix Angular 21 warning NG0956 by switching item tracking from object identity to stable item.uid, aligned with vscroll persistent item identity updates.

Also expands adapter test coverage to validate uid uniqueness and stability across operations like update/append/prepend/clip/reset/reload/remove.

[dhilt]

On issue #364 (track by identity warning).

Plan for possible fixes

1. Optional external trackBy

Add a config for custom trackBy (on datasource?).
Only used when explicitly enabled (not default).

2. Stabilize Item object identity in core

Goal: minimize recreation of Item objects.
Do a buffer diff and reuse existing Items, mutating $index, data, size instead of creating new ones.
Touches vscroll core logic, higher regression risk.

3. Introduce a stable uid on Item

Add a uid field and use track item.uid.
uid must survive “logical continuity” across cycles.
Requires a mechanism to persist/transfer uid (see next item).

4. Maintain a uid registry by datasource position

Keep a map index -> uid.
Adjust the map on insert/remove/update with proper shifting (fixRight aware).
Gives a stable uid per logical position, independent of user data.
Risk: if data changes at the same index, uid stays the same (position identity, not data identity).