Enhance ThemedTable2 with performance improvements and tests (#128)

This pull request focuses on improving the reliability, performance, and
test coverage of the `ThemedTable2` widget in the `layrz_theme` Flutter
library. The changes address critical issues like column key collisions,
update loss during loading, and isolate safety for closures, while also
optimizing multi-select and widget update performance. All debug logging
has been removed from the core filtering and sorting logic, and
comprehensive widget tests have been added.

**Bug fixes:**

* Fixed column key collision in `_itemsStrings` by using column index
instead of `col.hashCode`, preventing silent data corruption when
columns share the same `headerText`.
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* Fixed silent update loss: updates arriving while loading are now
queued via a `_pendingUpdate` flag and processed after the current
operation completes.
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* Fixed unsendable closure crash: sort keys are now precomputed on the
main thread before `compute()`, ensuring `valueBuilder` closures that
capture `BuildContext` or i18n objects never cross the isolate boundary.
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**Performance improvements:**

* Improved multi-select performance by introducing an internal `Set<T>`
mirroring `_selectedItems` for O(1) `contains()` lookups, replacing
previous O(n) checks.
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* Improved `didUpdateWidget` performance by replacing
`DeepCollectionEquality` (O(n)) with a fast O(1) heuristic using
`identical` and length checks.
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**Codebase cleanup:**

* Removed all `debugPrint` statements from `_filterAndSort` and related
logic.
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**Testing and versioning:**

* Added comprehensive widget tests for `ThemedTable2`, including
regression coverage for key collision, search, sorting, refresh, and
widget update scenarios.
* Bumped version to `7.5.22` in `pubspec.yaml` and
`.claude-plugin/plugin.json`.
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Author: ManuelRomeroA

.claude-plugin/plugin.json

@@ -1,5 +1,5 @@
 {
   "name": "layrz_theme",
   "description": "Claude Code skills for layrz_theme Flutter widget library",
-  "version": "7.5.21"
+  "version": "7.5.22"
 }