perf: Reduce allocations and redundant enumerations across Carousel, NumericEntry, Calendar, and BottomSheet

[PaulAndersonS]

Root Cause of the Issue

Multiple controls perform redundant work in frequently-called paths — repeated IEnumerable enumeration, per-character string allocations, per-instance collection allocations for defaults, and double dictionary lookups — all of which create unnecessary GC pressure.

Description of Change

Five targeted performance improvements across 4 controls:

1. Carousel (iOS/Windows/Android) — MapSelectedIndex/UpdateSelectedIndex called .Count() up to 3 times and .Any() once on IEnumerable<object> ItemsSource. Each call fully enumerates the collection. Fixed by caching the count in a local variable.

2. NumericEntry — GetMaximumFractionDigit built a digit string character-by-character using += (O(n²) allocations). Since the format suffix is at most 2 characters, replaced with a single int.TryParse() call that avoids all intermediate string allocations.

3. Calendar ViewChangedEventArgs — Each instance allocated new List<DateTime>().AsReadOnly() twice for default property values (2 List + 2 ReadOnlyCollection objects per event). Replaced with a shared static readonly empty collection.

4. Calendar CalendarViewHelper.GetCurrentMonthDates — new List<DateTime>() without capacity hint caused repeated internal array resizing. Pre-allocated with visibleDates.Count as the upper bound.

5. BottomSheet — ContainsKey + indexer SET pattern performs two hash lookups. Replaced with TryAdd() which performs a single lookup.

Issues Fixed

N/A — proactive performance improvement

Unit Tests Added

• CustomFormat_StandardFormatWithDigits_ParsesMaxFractionCorrectly — Verifies standard format specifiers (C2, C10, N0, F99) are parsed correctly after the int.TryParse optimization
• CustomFormat_CustomFormatWithNonDigitSuffix_FallsBackToCustomParsing — Verifies non-numeric suffixes fall back to custom format parsing
• CalendarViewChangedEventArgs_DefaultProperties_UseSharedEmptyCollection — Verifies the static empty collection is shared across instances (no per-instance allocation)
• CalendarViewChangedEventArgs_AssignedDates_ReturnsCorrectValues — Verifies assigned dates work correctly
• SelectedIndex_WithItemsSource_ClampsCorrectly — Verifies carousel index clamping with the cached count approach

Screenshots

N/A — no visual changes